* [PULL 0/8] Misc patches for QEMU 5.0-rc3
@ 2020-04-11 13:03 Paolo Bonzini
2020-04-11 13:03 ` [PULL 1/8] piix: fix xenfv regression, add compat machine xenfv-4.2 Paolo Bonzini
` (9 more replies)
0 siblings, 10 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel
The following changes since commit 53ef8a92eb04ee19640f5aad3bff36cd4a36c250:
Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20200406' into staging (2020-04-06 12:36:45 +0100)
are available in the Git repository at:
git://github.com/bonzini/qemu.git tags/for-upstream
for you to fetch changes up to 8c12bd8e32faf8c483cd96c1557899120bc67eea:
module: increase dirs array size by one (2020-04-11 08:49:26 -0400)
----------------------------------------------------------------
Bugfixes, and reworking of the atomics documentation.
----------------------------------------------------------------
Alexander Duyck (1):
memory: Do not allow direct write access to rom_device regions
Bauerchen (1):
oslib-posix: take lock before qemu_cond_broadcast
Bruce Rogers (1):
module: increase dirs array size by one
Igor Mammedov (1):
vl.c: error out if -mem-path is used together with -M memory-backend
Olaf Hering (1):
piix: fix xenfv regression, add compat machine xenfv-4.2
Paolo Bonzini (3):
atomics: convert to reStructuredText
atomics: update documentation
rcu: do not mention atomic_mb_read/set in documentation
docs/devel/atomics.rst | 507 +++++++++++++++++++++++++++++++++++
docs/devel/atomics.txt | 403 ----------------------------
docs/devel/index.rst | 1 +
docs/devel/rcu.txt | 4 +-
hw/i386/pc_piix.c | 19 +-
include/exec/memory.h | 4 +-
roms/SLOF | 2 +-
softmmu/vl.c | 5 +
tests/qtest/device-introspect-test.c | 2 +-
tests/qtest/qom-test.c | 42 +--
tests/qtest/test-hmp.c | 2 +-
util/module.c | 2 +-
util/oslib-posix.c | 3 +
13 files changed, 548 insertions(+), 448 deletions(-)
create mode 100644 docs/devel/atomics.rst
delete mode 100644 docs/devel/atomics.txt
--
2.18.2
^ permalink raw reply [flat|nested] 11+ messages in thread
* [PULL 1/8] piix: fix xenfv regression, add compat machine xenfv-4.2
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 2/8] oslib-posix: take lock before qemu_cond_broadcast Paolo Bonzini
` (8 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Olaf Hering
From: Olaf Hering <olaf@aepfle.de>
With QEMU 4.0 an incompatible change was added to pc_piix, which makes it
practical impossible to migrate domUs started with qemu2 or qemu3 to
newer qemu versions. Commit 7fccf2a06890e3bc3b30e29827ad3fb93fe88fea
added and enabled a new member "smbus_no_migration_support". In commit
4ab2f2a8aabfea95cc53c64e13b3f67960b27fdf the vmstate_acpi got new
elements, which are conditionally filled. As a result, an incoming
migration expected smbus related data unless smbus migration was
disabled for a given MachineClass. Since first commit forgot to handle
'xenfv', domUs started with QEMU 4.x are incompatible with their QEMU
siblings.
Using other existing machine types, such as 'pc-i440fx-3.1', is not
possible because 'xenfv' creates the 'xen-platform' PCI device at
00:02.0, while all other variants to run a domU would create it at
00:04.0.
To cover both the existing and the broken case of 'xenfv' in a single
qemu binary, a new compatibility variant of 'xenfv-4.2' must be added
which targets domUs started with qemu 4.2. The existing 'xenfv' restores
compatibility of QEMU 5.x with qemu 3.1.
Host admins who started domUs with QEMU 4.x (preferrable QEMU 4.2)
have to use a wrapper script which appends '-machine xenfv-4.2' to
the device-model command line. This is only required if there is no
maintenance window which allows to temporary shutdown the domU and
restart it with a fixed device-model.
The wrapper script is as simple as this:
#!/bin/sh
exec /usr/bin/qemu-system-i386 "$@" -machine xenfv-4.2
With xl this script will be enabled with device_model_override=, see
xl.cfg(5). To live migrate a domU, adjust the existing domU.cfg and pass
it to xl migrate or xl save/restore:
xl migrate -C new-domU.cfg domU remote-host
xl save domU CheckpointFile new-domU.cfg
xl restore new-domU.cfg CheckpointFile
With libvirt this script will be enabled with the <emulator> element in
domU.xml. Use 'virsh edit' prior 'virsh migrate' to replace the existing
<emulator> element to point it to the wrapper script.
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Message-Id: <20200327151841.13877-1-olaf@aepfle.de>
[Adjust tests for blacklisted machine types, simplifying the one in
qom-test. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
hw/i386/pc_piix.c | 19 +++++++++++--
tests/qtest/device-introspect-test.c | 2 +-
tests/qtest/qom-test.c | 42 ++++++----------------------
tests/qtest/test-hmp.c | 2 +-
4 files changed, 26 insertions(+), 39 deletions(-)
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 9cceae3e2c..22dee0e76c 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -948,13 +948,26 @@ DEFINE_PC_MACHINE(isapc, "isapc", pc_init_isa,
#ifdef CONFIG_XEN
-static void xenfv_machine_options(MachineClass *m)
+static void xenfv_4_2_machine_options(MachineClass *m)
{
+ pc_i440fx_4_2_machine_options(m);
+ m->desc = "Xen Fully-virtualized PC";
+ m->max_cpus = HVM_MAX_VCPUS;
+ m->default_machine_opts = "accel=xen";
+}
+
+DEFINE_PC_MACHINE(xenfv_4_2, "xenfv-4.2", pc_xen_hvm_init,
+ xenfv_4_2_machine_options);
+
+static void xenfv_3_1_machine_options(MachineClass *m)
+{
+ pc_i440fx_3_1_machine_options(m);
m->desc = "Xen Fully-virtualized PC";
+ m->alias = "xenfv";
m->max_cpus = HVM_MAX_VCPUS;
m->default_machine_opts = "accel=xen";
}
-DEFINE_PC_MACHINE(xenfv, "xenfv", pc_xen_hvm_init,
- xenfv_machine_options);
+DEFINE_PC_MACHINE(xenfv, "xenfv-3.1", pc_xen_hvm_init,
+ xenfv_3_1_machine_options);
#endif
diff --git a/tests/qtest/device-introspect-test.c b/tests/qtest/device-introspect-test.c
index 04f22903b0..f2c1576cae 100644
--- a/tests/qtest/device-introspect-test.c
+++ b/tests/qtest/device-introspect-test.c
@@ -288,7 +288,7 @@ static void add_machine_test_case(const char *mname)
char *path, *args;
/* Ignore blacklisted machines */
- if (g_str_equal("xenfv", mname) || g_str_equal("xenpv", mname)) {
+ if (!memcmp("xenfv", mname, 5) || g_str_equal("xenpv", mname)) {
return;
}
diff --git a/tests/qtest/qom-test.c b/tests/qtest/qom-test.c
index 4f94cc678c..e338a41194 100644
--- a/tests/qtest/qom-test.c
+++ b/tests/qtest/qom-test.c
@@ -15,35 +15,6 @@
#include "qemu/cutils.h"
#include "libqtest.h"
-static const char *blacklist_x86[] = {
- "xenfv", "xenpv", NULL
-};
-
-static const struct {
- const char *arch;
- const char **machine;
-} blacklists[] = {
- { "i386", blacklist_x86 },
- { "x86_64", blacklist_x86 },
-};
-
-static bool is_blacklisted(const char *arch, const char *mach)
-{
- int i;
- const char **p;
-
- for (i = 0; i < ARRAY_SIZE(blacklists); i++) {
- if (!strcmp(blacklists[i].arch, arch)) {
- for (p = blacklists[i].machine; *p; p++) {
- if (!strcmp(*p, mach)) {
- return true;
- }
- }
- }
- }
- return false;
-}
-
static void test_properties(QTestState *qts, const char *path, bool recurse)
{
char *child_path;
@@ -108,13 +79,16 @@ static void test_machine(gconstpointer data)
static void add_machine_test_case(const char *mname)
{
- const char *arch = qtest_get_arch();
+ char *path;
- if (!is_blacklisted(arch, mname)) {
- char *path = g_strdup_printf("qom/%s", mname);
- qtest_add_data_func(path, g_strdup(mname), test_machine);
- g_free(path);
+ /* Ignore blacklisted machines that have known problems */
+ if (!memcmp("xenfv", mname, 5) || g_str_equal("xenpv", mname)) {
+ return;
}
+
+ path = g_strdup_printf("qom/%s", mname);
+ qtest_add_data_func(path, g_strdup(mname), test_machine);
+ g_free(path);
}
int main(int argc, char **argv)
diff --git a/tests/qtest/test-hmp.c b/tests/qtest/test-hmp.c
index 5029c4d2c9..f8aa5f92c5 100644
--- a/tests/qtest/test-hmp.c
+++ b/tests/qtest/test-hmp.c
@@ -143,7 +143,7 @@ static void add_machine_test_case(const char *mname)
char *path;
/* Ignore blacklisted machines that have known problems */
- if (!strcmp("xenfv", mname) || !strcmp("xenpv", mname)) {
+ if (!memcmp("xenfv", mname, 5) || g_str_equal("xenpv", mname)) {
return;
}
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 2/8] oslib-posix: take lock before qemu_cond_broadcast
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
2020-04-11 13:03 ` [PULL 1/8] piix: fix xenfv regression, add compat machine xenfv-4.2 Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 3/8] atomics: convert to reStructuredText Paolo Bonzini
` (7 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Bauerchen
From: Bauerchen <bauerchen@tencent.com>
In touch_all_pages, if the mutex is not taken around qemu_cond_broadcast,
qemu_cond_broadcast may be called before all touch page threads enter
qemu_cond_wait. In this case, the touch page threads wait forever for the
main thread to wake them up, causing a deadlock.
Signed-off-by: Bauerchen <bauerchen@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
util/oslib-posix.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 4dd6d7d4b4..062236a1ab 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -492,8 +492,11 @@ static bool touch_all_pages(char *area, size_t hpagesize, size_t numpages,
QEMU_THREAD_JOINABLE);
addr += memset_thread[i].numpages * hpagesize;
}
+
+ qemu_mutex_lock(&page_mutex);
threads_created_flag = true;
qemu_cond_broadcast(&page_cond);
+ qemu_mutex_unlock(&page_mutex);
for (i = 0; i < memset_num_threads; i++) {
qemu_thread_join(&memset_thread[i].pgthread);
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 3/8] atomics: convert to reStructuredText
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
2020-04-11 13:03 ` [PULL 1/8] piix: fix xenfv regression, add compat machine xenfv-4.2 Paolo Bonzini
2020-04-11 13:03 ` [PULL 2/8] oslib-posix: take lock before qemu_cond_broadcast Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 4/8] atomics: update documentation Paolo Bonzini
` (6 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel
No attempts to fix or update the text; these are left for the next
patch in the series.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
docs/devel/atomics.rst | 446 +++++++++++++++++++++++++++++++++++++++++
docs/devel/atomics.txt | 403 -------------------------------------
docs/devel/index.rst | 1 +
3 files changed, 447 insertions(+), 403 deletions(-)
create mode 100644 docs/devel/atomics.rst
delete mode 100644 docs/devel/atomics.txt
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst
new file mode 100644
index 0000000000..83ed3d6981
--- /dev/null
+++ b/docs/devel/atomics.rst
@@ -0,0 +1,446 @@
+=========================
+Atomic operations in QEMU
+=========================
+
+CPUs perform independent memory operations effectively in random order.
+but this can be a problem for CPU-CPU interaction (including interactions
+between QEMU and the guest). Multi-threaded programs use various tools
+to instruct the compiler and the CPU to restrict the order to something
+that is consistent with the expectations of the programmer.
+
+The most basic tool is locking. Mutexes, condition variables and
+semaphores are used in QEMU, and should be the default approach to
+synchronization. Anything else is considerably harder, but it's
+also justified more often than one would like. The two tools that
+are provided by ``qemu/atomic.h`` are memory barriers and atomic operations.
+
+Macros defined by ``qemu/atomic.h`` fall in three camps:
+
+- compiler barriers: ``barrier()``;
+
+- weak atomic access and manual memory barriers: ``atomic_read()``,
+ ``atomic_set()``, ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``, ``smp_mb_acquire()``,
+ ``smp_mb_release()``, ``smp_read_barrier_depends()``;
+
+- sequentially consistent atomic access: everything else.
+
+
+Compiler memory barrier
+=======================
+
+``barrier()`` prevents the compiler from moving the memory accesses either
+side of it to the other side. The compiler barrier has no direct effect
+on the CPU, which may then reorder things however it wishes.
+
+``barrier()`` is mostly used within ``qemu/atomic.h`` itself. On some
+architectures, CPU guarantees are strong enough that blocking compiler
+optimizations already ensures the correct order of execution. In this
+case, ``qemu/atomic.h`` will reduce stronger memory barriers to simple
+compiler barriers.
+
+Still, ``barrier()`` can be useful when writing code that can be interrupted
+by signal handlers.
+
+
+Sequentially consistent atomic access
+=====================================
+
+Most of the operations in the ``qemu/atomic.h`` header ensure *sequential
+consistency*, where "the result of any execution is the same as if the
+operations of all the processors were executed in some sequential order,
+and the operations of each individual processor appear in this sequence
+in the order specified by its program".
+
+``qemu/atomic.h`` provides the following set of atomic read-modify-write
+operations::
+
+ void atomic_inc(ptr)
+ void atomic_dec(ptr)
+ void atomic_add(ptr, val)
+ void atomic_sub(ptr, val)
+ void atomic_and(ptr, val)
+ void atomic_or(ptr, val)
+
+ typeof(*ptr) atomic_fetch_inc(ptr)
+ typeof(*ptr) atomic_fetch_dec(ptr)
+ typeof(*ptr) atomic_fetch_add(ptr, val)
+ typeof(*ptr) atomic_fetch_sub(ptr, val)
+ typeof(*ptr) atomic_fetch_and(ptr, val)
+ typeof(*ptr) atomic_fetch_or(ptr, val)
+ typeof(*ptr) atomic_fetch_xor(ptr, val)
+ typeof(*ptr) atomic_fetch_inc_nonzero(ptr)
+ typeof(*ptr) atomic_xchg(ptr, val)
+ typeof(*ptr) atomic_cmpxchg(ptr, old, new)
+
+all of which return the old value of ``*ptr``. These operations are
+polymorphic; they operate on any type that is as wide as a pointer.
+
+Similar operations return the new value of ``*ptr``::
+
+ typeof(*ptr) atomic_inc_fetch(ptr)
+ typeof(*ptr) atomic_dec_fetch(ptr)
+ typeof(*ptr) atomic_add_fetch(ptr, val)
+ typeof(*ptr) atomic_sub_fetch(ptr, val)
+ typeof(*ptr) atomic_and_fetch(ptr, val)
+ typeof(*ptr) atomic_or_fetch(ptr, val)
+ typeof(*ptr) atomic_xor_fetch(ptr, val)
+
+Sequentially consistent loads and stores can be done using::
+
+ atomic_fetch_add(ptr, 0) for loads
+ atomic_xchg(ptr, val) for stores
+
+However, they are quite expensive on some platforms, notably POWER and
+Arm. Therefore, qemu/atomic.h provides two primitives with slightly
+weaker constraints::
+
+ typeof(*ptr) atomic_mb_read(ptr)
+ void atomic_mb_set(ptr, val)
+
+The semantics of these primitives map to Java volatile variables,
+and are strongly related to memory barriers as used in the Linux
+kernel (see below).
+
+As long as you use atomic_mb_read and atomic_mb_set, accesses cannot
+be reordered with each other, and it is also not possible to reorder
+"normal" accesses around them.
+
+However, and this is the important difference between
+atomic_mb_read/atomic_mb_set and sequential consistency, it is important
+for both threads to access the same volatile variable. It is not the
+case that everything visible to thread A when it writes volatile field f
+becomes visible to thread B after it reads volatile field g. The store
+and load have to "match" (i.e., be performed on the same volatile
+field) to achieve the right semantics.
+
+
+These operations operate on any type that is as wide as an int or smaller.
+
+
+Weak atomic access and manual memory barriers
+=============================================
+
+Compared to sequentially consistent atomic access, programming with
+weaker consistency models can be considerably more complicated.
+In general, if the algorithm you are writing includes both writes
+and reads on the same side, it is generally simpler to use sequentially
+consistent primitives.
+
+When using this model, variables are accessed with:
+
+- ``atomic_read()`` and ``atomic_set()``; these prevent the compiler from
+ optimizing accesses out of existence and creating unsolicited
+ accesses, but do not otherwise impose any ordering on loads and
+ stores: both the compiler and the processor are free to reorder
+ them.
+
+- ``atomic_load_acquire()``, which guarantees the LOAD to appear to
+ happen, with respect to the other components of the system,
+ before all the LOAD or STORE operations specified afterwards.
+ Operations coming before ``atomic_load_acquire()`` can still be
+ reordered after it.
+
+- ``atomic_store_release()``, which guarantees the STORE to appear to
+ happen, with respect to the other components of the system,
+ after all the LOAD or STORE operations specified afterwards.
+ Operations coming after ``atomic_store_release()`` can still be
+ reordered after it.
+
+Restrictions to the ordering of accesses can also be specified
+using the memory barrier macros: ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
+``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``.
+
+Memory barriers control the order of references to shared memory.
+They come in six kinds:
+
+- ``smp_rmb()`` guarantees that all the LOAD operations specified before
+ the barrier will appear to happen before all the LOAD operations
+ specified after the barrier with respect to the other components of
+ the system.
+
+ In other words, ``smp_rmb()`` puts a partial ordering on loads, but is not
+ required to have any effect on stores.
+
+- ``smp_wmb()`` guarantees that all the STORE operations specified before
+ the barrier will appear to happen before all the STORE operations
+ specified after the barrier with respect to the other components of
+ the system.
+
+ In other words, ``smp_wmb()`` puts a partial ordering on stores, but is not
+ required to have any effect on loads.
+
+- ``smp_mb_acquire()`` guarantees that all the LOAD operations specified before
+ the barrier will appear to happen before all the LOAD or STORE operations
+ specified after the barrier with respect to the other components of
+ the system.
+
+- ``smp_mb_release()`` guarantees that all the STORE operations specified *after*
+ the barrier will appear to happen after all the LOAD or STORE operations
+ specified *before* the barrier with respect to the other components of
+ the system.
+
+- ``smp_mb()`` guarantees that all the LOAD and STORE operations specified
+ before the barrier will appear to happen before all the LOAD and
+ STORE operations specified after the barrier with respect to the other
+ components of the system.
+
+ ``smp_mb()`` puts a partial ordering on both loads and stores. It is
+ stronger than both a read and a write memory barrier; it implies both
+ ``smp_mb_acquire()`` and ``smp_mb_release()``, but it also prevents STOREs
+ coming before the barrier from overtaking LOADs coming after the
+ barrier and vice versa.
+
+- ``smp_read_barrier_depends()`` is a weaker kind of read barrier. On
+ most processors, whenever two loads are performed such that the
+ second depends on the result of the first (e.g., the first load
+ retrieves the address to which the second load will be directed),
+ the processor will guarantee that the first LOAD will appear to happen
+ before the second with respect to the other components of the system.
+ However, this is not always true---for example, it was not true on
+ Alpha processors. Whenever this kind of access happens to shared
+ memory (that is not protected by a lock), a read barrier is needed,
+ and ``smp_read_barrier_depends()`` can be used instead of ``smp_rmb()``.
+
+ Note that the first load really has to have a _data_ dependency and not
+ a control dependency. If the address for the second load is dependent
+ on the first load, but the dependency is through a conditional rather
+ than actually loading the address itself, then it's a _control_
+ dependency and a full read barrier or better is required.
+
+
+This is the set of barriers that is required *between* two ``atomic_read()``
+and ``atomic_set()`` operations to achieve sequential consistency:
+
+ +----------------+-------------------------------------------------------+
+ | | 2nd operation |
+ | +------------------+-----------------+------------------+
+ | 1st operation | (after last) | atomic_read | atomic_set |
+ +----------------+------------------+-----------------+------------------+
+ | (before first) | .. | none | smp_mb_release() |
+ +----------------+------------------+-----------------+------------------+
+ | atomic_read | smp_mb_acquire() | smp_rmb() [1]_ | [2]_ |
+ +----------------+------------------+-----------------+------------------+
+ | atomic_set | none | smp_mb() [3]_ | smp_wmb() |
+ +----------------+------------------+-----------------+------------------+
+
+ .. [1] Or smp_read_barrier_depends().
+
+ .. [2] This requires a load-store barrier. This is achieved by
+ either smp_mb_acquire() or smp_mb_release().
+
+ .. [3] This requires a store-load barrier. On most machines, the only
+ way to achieve this is a full barrier.
+
+
+You can see that the two possible definitions of ``atomic_mb_read()``
+and ``atomic_mb_set()`` are the following:
+
+ 1) | atomic_mb_read(p) = atomic_read(p); smp_mb_acquire()
+ | atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v); smp_mb()
+
+ 2) | atomic_mb_read(p) = smp_mb() atomic_read(p); smp_mb_acquire()
+ | atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v);
+
+Usually the former is used, because ``smp_mb()`` is expensive and a program
+normally has more reads than writes. Therefore it makes more sense to
+make ``atomic_mb_set()`` the more expensive operation.
+
+There are two common cases in which atomic_mb_read and atomic_mb_set
+generate too many memory barriers, and thus it can be useful to manually
+place barriers, or use atomic_load_acquire/atomic_store_release instead:
+
+- when a data structure has one thread that is always a writer
+ and one thread that is always a reader, manual placement of
+ memory barriers makes the write side faster. Furthermore,
+ correctness is easy to check for in this case using the "pairing"
+ trick that is explained below:
+
+ +----------------------------------------------------------------------+
+ | thread 1 |
+ +-----------------------------------+----------------------------------+
+ | before | after |
+ +===================================+==================================+
+ | :: | :: |
+ | | |
+ | (other writes) | |
+ | atomic_mb_set(&a, x) | atomic_store_release(&a, x) |
+ | atomic_mb_set(&b, y) | atomic_store_release(&b, y) |
+ +-----------------------------------+----------------------------------+
+
+ +----------------------------------------------------------------------+
+ | thread 2 |
+ +-----------------------------------+----------------------------------+
+ | before | after |
+ +===================================+==================================+
+ | :: | :: |
+ | | |
+ | y = atomic_mb_read(&b) | y = atomic_load_acquire(&b) |
+ | x = atomic_mb_read(&a) | x = atomic_load_acquire(&a) |
+ | (other reads) | |
+ +-----------------------------------+----------------------------------+
+
+ Note that the barrier between the stores in thread 1, and between
+ the loads in thread 2, has been optimized here to a write or a
+ read memory barrier respectively. On some architectures, notably
+ ARMv7, smp_mb_acquire and smp_mb_release are just as expensive as
+ smp_mb, but smp_rmb and/or smp_wmb are more efficient.
+
+- sometimes, a thread is accessing many variables that are otherwise
+ unrelated to each other (for example because, apart from the current
+ thread, exactly one other thread will read or write each of these
+ variables). In this case, it is possible to "hoist" the implicit
+ barriers provided by ``atomic_mb_read()`` and ``atomic_mb_set()`` outside
+ a loop. For example, the above definition ``atomic_mb_read()`` gives
+ the following transformation:
+
+ +-----------------------------------+----------------------------------+
+ | before | after |
+ +===================================+==================================+
+ | :: | :: |
+ | | |
+ | n = 0; | n = 0; |
+ | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
+ | n += atomic_mb_read(&a[i]); | n += atomic_read(&a[i]); |
+ | | smp_mb_acquire(); |
+ +-----------------------------------+----------------------------------+
+
+ Similarly, atomic_mb_set() can be transformed as follows:
+
+ +-----------------------------------+----------------------------------+
+ | before | after |
+ +===================================+==================================+
+ | :: | :: |
+ | | |
+ | | smp_mb_release(); |
+ | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
+ | atomic_mb_set(&a[i], false); | atomic_set(&a[i], false); |
+ | | smp_mb(); |
+ +-----------------------------------+----------------------------------+
+
+
+ The other thread can still use ``atomic_mb_read()``/``atomic_mb_set()``.
+
+The two tricks can be combined. In this case, splitting a loop in
+two lets you hoist the barriers out of the loops _and_ eliminate the
+expensive ``smp_mb()``:
+
+ +-----------------------------------+----------------------------------+
+ | before | after |
+ +===================================+==================================+
+ | :: | :: |
+ | | |
+ | | smp_mb_release(); |
+ | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
+ | atomic_mb_set(&a[i], false); | atomic_set(&a[i], false); |
+ | atomic_mb_set(&b[i], false); | smb_wmb(); |
+ | } | for (i = 0; i < 10; i++) |
+ | | atomic_set(&a[i], false); |
+ | | smp_mb(); |
+ +-----------------------------------+----------------------------------+
+
+
+Memory barrier pairing
+----------------------
+
+A useful rule of thumb is that memory barriers should always, or almost
+always, be paired with another barrier. In the case of QEMU, however,
+note that the other barrier may actually be in a driver that runs in
+the guest!
+
+For the purposes of pairing, ``smp_read_barrier_depends()`` and ``smp_rmb()``
+both count as read barriers. A read barrier shall pair with a write
+barrier or a full barrier; a write barrier shall pair with a read
+barrier or a full barrier. A full barrier can pair with anything.
+For example:
+
+ +--------------------+------------------------------+
+ | thread 1 | thread 2 |
+ +====================+==============================+
+ | :: | :: |
+ | | |
+ | a = 1; | |
+ | smp_wmb(); | |
+ | b = 2; | x = b; |
+ | | smp_rmb(); |
+ | | y = a; |
+ +--------------------+------------------------------+
+
+Note that the "writing" thread is accessing the variables in the
+opposite order as the "reading" thread. This is expected: stores
+before the write barrier will normally match the loads after the
+read barrier, and vice versa. The same is true for more than 2
+access and for data dependency barriers:
+
+ +----------------------+------------------------------+
+ | thread 1 | thread 2 |
+ +======================+==============================+
+ | :: | :: |
+ | | |
+ | b[2] = 1; | |
+ | smp_wmb(); | |
+ | x->i = 2; | |
+ | smp_wmb(); | |
+ | a = x; | x = a; |
+ | | smp_read_barrier_depends(); |
+ | | y = x->i; |
+ | | smp_read_barrier_depends(); |
+ | | z = b[y]; |
+ +----------------------+------------------------------+
+
+``smp_wmb()`` also pairs with ``atomic_mb_read()`` and ``smp_mb_acquire()``.
+and ``smp_rmb()`` also pairs with ``atomic_mb_set()`` and ``smp_mb_release()``.
+
+
+Comparison with Linux kernel memory barriers
+============================================
+
+Here is a list of differences between Linux kernel atomic operations
+and memory barriers, and the equivalents in QEMU:
+
+- atomic operations in Linux are always on a 32-bit int type and
+ use a boxed ``atomic_t`` type; atomic operations in QEMU are polymorphic
+ and use normal C types.
+
+- Originally, ``atomic_read`` and ``atomic_set`` in Linux gave no guarantee
+ at all. Linux 4.1 updated them to implement volatile
+ semantics via ``ACCESS_ONCE`` (or the more recent ``READ``/``WRITE_ONCE``).
+
+ QEMU's ``atomic_read`` and ``atomic_set`` implement C11 atomic relaxed
+ semantics if the compiler supports it, and volatile semantics otherwise.
+ Both semantics prevent the compiler from doing certain transformations;
+ the difference is that atomic accesses are guaranteed to be atomic,
+ while volatile accesses aren't. Thus, in the volatile case we just cross
+ our fingers hoping that the compiler will generate atomic accesses,
+ since we assume the variables passed are machine-word sized and
+ properly aligned.
+
+ No barriers are implied by ``atomic_read`` and ``atomic_set`` in either Linux
+ or QEMU.
+
+- atomic read-modify-write operations in Linux are of three kinds:
+
+ ===================== =========================================
+ ``atomic_OP`` returns void
+ ``atomic_OP_return`` returns new value of the variable
+ ``atomic_fetch_OP`` returns the old value of the variable
+ ``atomic_cmpxchg`` returns the old value of the variable
+ ===================== =========================================
+
+ In QEMU, the second kind does not exist. Currently Linux has
+ atomic_fetch_or only. QEMU provides and, or, inc, dec, add, sub.
+
+- different atomic read-modify-write operations in Linux imply
+ a different set of memory barriers; in QEMU, all of them enforce
+ sequential consistency, which means they imply full memory barriers
+ before and after the operation.
+
+- Linux does not have an equivalent of ``atomic_mb_set()``. In particular,
+ note that ``smp_store_mb()`` is a little weaker than ``atomic_mb_set()``.
+ ``atomic_mb_read()`` compiles to the same instructions as Linux's
+ ``smp_load_acquire()``, but this should be treated as an implementation
+ detail.
+
+Sources
+=======
+
+- ``Documentation/memory-barriers.txt`` from the Linux kernel
diff --git a/docs/devel/atomics.txt b/docs/devel/atomics.txt
deleted file mode 100644
index 67bdf82628..0000000000
--- a/docs/devel/atomics.txt
+++ /dev/null
@@ -1,403 +0,0 @@
-CPUs perform independent memory operations effectively in random order.
-but this can be a problem for CPU-CPU interaction (including interactions
-between QEMU and the guest). Multi-threaded programs use various tools
-to instruct the compiler and the CPU to restrict the order to something
-that is consistent with the expectations of the programmer.
-
-The most basic tool is locking. Mutexes, condition variables and
-semaphores are used in QEMU, and should be the default approach to
-synchronization. Anything else is considerably harder, but it's
-also justified more often than one would like. The two tools that
-are provided by qemu/atomic.h are memory barriers and atomic operations.
-
-Macros defined by qemu/atomic.h fall in three camps:
-
-- compiler barriers: barrier();
-
-- weak atomic access and manual memory barriers: atomic_read(),
- atomic_set(), smp_rmb(), smp_wmb(), smp_mb(), smp_mb_acquire(),
- smp_mb_release(), smp_read_barrier_depends();
-
-- sequentially consistent atomic access: everything else.
-
-
-COMPILER MEMORY BARRIER
-=======================
-
-barrier() prevents the compiler from moving the memory accesses either
-side of it to the other side. The compiler barrier has no direct effect
-on the CPU, which may then reorder things however it wishes.
-
-barrier() is mostly used within qemu/atomic.h itself. On some
-architectures, CPU guarantees are strong enough that blocking compiler
-optimizations already ensures the correct order of execution. In this
-case, qemu/atomic.h will reduce stronger memory barriers to simple
-compiler barriers.
-
-Still, barrier() can be useful when writing code that can be interrupted
-by signal handlers.
-
-
-SEQUENTIALLY CONSISTENT ATOMIC ACCESS
-=====================================
-
-Most of the operations in the qemu/atomic.h header ensure *sequential
-consistency*, where "the result of any execution is the same as if the
-operations of all the processors were executed in some sequential order,
-and the operations of each individual processor appear in this sequence
-in the order specified by its program".
-
-qemu/atomic.h provides the following set of atomic read-modify-write
-operations:
-
- void atomic_inc(ptr)
- void atomic_dec(ptr)
- void atomic_add(ptr, val)
- void atomic_sub(ptr, val)
- void atomic_and(ptr, val)
- void atomic_or(ptr, val)
-
- typeof(*ptr) atomic_fetch_inc(ptr)
- typeof(*ptr) atomic_fetch_dec(ptr)
- typeof(*ptr) atomic_fetch_add(ptr, val)
- typeof(*ptr) atomic_fetch_sub(ptr, val)
- typeof(*ptr) atomic_fetch_and(ptr, val)
- typeof(*ptr) atomic_fetch_or(ptr, val)
- typeof(*ptr) atomic_fetch_xor(ptr, val)
- typeof(*ptr) atomic_fetch_inc_nonzero(ptr)
- typeof(*ptr) atomic_xchg(ptr, val)
- typeof(*ptr) atomic_cmpxchg(ptr, old, new)
-
-all of which return the old value of *ptr. These operations are
-polymorphic; they operate on any type that is as wide as a pointer.
-
-Similar operations return the new value of *ptr:
-
- typeof(*ptr) atomic_inc_fetch(ptr)
- typeof(*ptr) atomic_dec_fetch(ptr)
- typeof(*ptr) atomic_add_fetch(ptr, val)
- typeof(*ptr) atomic_sub_fetch(ptr, val)
- typeof(*ptr) atomic_and_fetch(ptr, val)
- typeof(*ptr) atomic_or_fetch(ptr, val)
- typeof(*ptr) atomic_xor_fetch(ptr, val)
-
-Sequentially consistent loads and stores can be done using:
-
- atomic_fetch_add(ptr, 0) for loads
- atomic_xchg(ptr, val) for stores
-
-However, they are quite expensive on some platforms, notably POWER and
-Arm. Therefore, qemu/atomic.h provides two primitives with slightly
-weaker constraints:
-
- typeof(*ptr) atomic_mb_read(ptr)
- void atomic_mb_set(ptr, val)
-
-The semantics of these primitives map to Java volatile variables,
-and are strongly related to memory barriers as used in the Linux
-kernel (see below).
-
-As long as you use atomic_mb_read and atomic_mb_set, accesses cannot
-be reordered with each other, and it is also not possible to reorder
-"normal" accesses around them.
-
-However, and this is the important difference between
-atomic_mb_read/atomic_mb_set and sequential consistency, it is important
-for both threads to access the same volatile variable. It is not the
-case that everything visible to thread A when it writes volatile field f
-becomes visible to thread B after it reads volatile field g. The store
-and load have to "match" (i.e., be performed on the same volatile
-field) to achieve the right semantics.
-
-
-These operations operate on any type that is as wide as an int or smaller.
-
-
-WEAK ATOMIC ACCESS AND MANUAL MEMORY BARRIERS
-=============================================
-
-Compared to sequentially consistent atomic access, programming with
-weaker consistency models can be considerably more complicated.
-In general, if the algorithm you are writing includes both writes
-and reads on the same side, it is generally simpler to use sequentially
-consistent primitives.
-
-When using this model, variables are accessed with:
-
-- atomic_read() and atomic_set(); these prevent the compiler from
- optimizing accesses out of existence and creating unsolicited
- accesses, but do not otherwise impose any ordering on loads and
- stores: both the compiler and the processor are free to reorder
- them.
-
-- atomic_load_acquire(), which guarantees the LOAD to appear to
- happen, with respect to the other components of the system,
- before all the LOAD or STORE operations specified afterwards.
- Operations coming before atomic_load_acquire() can still be
- reordered after it.
-
-- atomic_store_release(), which guarantees the STORE to appear to
- happen, with respect to the other components of the system,
- after all the LOAD or STORE operations specified afterwards.
- Operations coming after atomic_store_release() can still be
- reordered after it.
-
-Restrictions to the ordering of accesses can also be specified
-using the memory barrier macros: smp_rmb(), smp_wmb(), smp_mb(),
-smp_mb_acquire(), smp_mb_release(), smp_read_barrier_depends().
-
-Memory barriers control the order of references to shared memory.
-They come in six kinds:
-
-- smp_rmb() guarantees that all the LOAD operations specified before
- the barrier will appear to happen before all the LOAD operations
- specified after the barrier with respect to the other components of
- the system.
-
- In other words, smp_rmb() puts a partial ordering on loads, but is not
- required to have any effect on stores.
-
-- smp_wmb() guarantees that all the STORE operations specified before
- the barrier will appear to happen before all the STORE operations
- specified after the barrier with respect to the other components of
- the system.
-
- In other words, smp_wmb() puts a partial ordering on stores, but is not
- required to have any effect on loads.
-
-- smp_mb_acquire() guarantees that all the LOAD operations specified before
- the barrier will appear to happen before all the LOAD or STORE operations
- specified after the barrier with respect to the other components of
- the system.
-
-- smp_mb_release() guarantees that all the STORE operations specified *after*
- the barrier will appear to happen after all the LOAD or STORE operations
- specified *before* the barrier with respect to the other components of
- the system.
-
-- smp_mb() guarantees that all the LOAD and STORE operations specified
- before the barrier will appear to happen before all the LOAD and
- STORE operations specified after the barrier with respect to the other
- components of the system.
-
- smp_mb() puts a partial ordering on both loads and stores. It is
- stronger than both a read and a write memory barrier; it implies both
- smp_mb_acquire() and smp_mb_release(), but it also prevents STOREs
- coming before the barrier from overtaking LOADs coming after the
- barrier and vice versa.
-
-- smp_read_barrier_depends() is a weaker kind of read barrier. On
- most processors, whenever two loads are performed such that the
- second depends on the result of the first (e.g., the first load
- retrieves the address to which the second load will be directed),
- the processor will guarantee that the first LOAD will appear to happen
- before the second with respect to the other components of the system.
- However, this is not always true---for example, it was not true on
- Alpha processors. Whenever this kind of access happens to shared
- memory (that is not protected by a lock), a read barrier is needed,
- and smp_read_barrier_depends() can be used instead of smp_rmb().
-
- Note that the first load really has to have a _data_ dependency and not
- a control dependency. If the address for the second load is dependent
- on the first load, but the dependency is through a conditional rather
- than actually loading the address itself, then it's a _control_
- dependency and a full read barrier or better is required.
-
-
-This is the set of barriers that is required *between* two atomic_read()
-and atomic_set() operations to achieve sequential consistency:
-
- | 2nd operation |
- |-----------------------------------------------|
- 1st operation | (after last) | atomic_read | atomic_set |
- ---------------+----------------+-------------+----------------|
- (before first) | | none | smp_mb_release |
- ---------------+----------------+-------------+----------------|
- atomic_read | smp_mb_acquire | smp_rmb | ** |
- ---------------+----------------+-------------+----------------|
- atomic_set | none | smp_mb()*** | smp_wmb() |
- ---------------+----------------+-------------+----------------|
-
- * Or smp_read_barrier_depends().
-
- ** This requires a load-store barrier. This is achieved by
- either smp_mb_acquire() or smp_mb_release().
-
- *** This requires a store-load barrier. On most machines, the only
- way to achieve this is a full barrier.
-
-
-You can see that the two possible definitions of atomic_mb_read()
-and atomic_mb_set() are the following:
-
- 1) atomic_mb_read(p) = atomic_read(p); smp_mb_acquire()
- atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v); smp_mb()
-
- 2) atomic_mb_read(p) = smp_mb() atomic_read(p); smp_mb_acquire()
- atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v);
-
-Usually the former is used, because smp_mb() is expensive and a program
-normally has more reads than writes. Therefore it makes more sense to
-make atomic_mb_set() the more expensive operation.
-
-There are two common cases in which atomic_mb_read and atomic_mb_set
-generate too many memory barriers, and thus it can be useful to manually
-place barriers, or use atomic_load_acquire/atomic_store_release instead:
-
-- when a data structure has one thread that is always a writer
- and one thread that is always a reader, manual placement of
- memory barriers makes the write side faster. Furthermore,
- correctness is easy to check for in this case using the "pairing"
- trick that is explained below:
-
- thread 1 thread 1
- ------------------------- ------------------------
- (other writes)
- atomic_mb_set(&a, x) atomic_store_release(&a, x)
- atomic_mb_set(&b, y) atomic_store_release(&b, y)
-
- =>
- thread 2 thread 2
- ------------------------- ------------------------
- y = atomic_mb_read(&b) y = atomic_load_acquire(&b)
- x = atomic_mb_read(&a) x = atomic_load_acquire(&a)
- (other reads)
-
- Note that the barrier between the stores in thread 1, and between
- the loads in thread 2, has been optimized here to a write or a
- read memory barrier respectively. On some architectures, notably
- ARMv7, smp_mb_acquire and smp_mb_release are just as expensive as
- smp_mb, but smp_rmb and/or smp_wmb are more efficient.
-
-- sometimes, a thread is accessing many variables that are otherwise
- unrelated to each other (for example because, apart from the current
- thread, exactly one other thread will read or write each of these
- variables). In this case, it is possible to "hoist" the implicit
- barriers provided by atomic_mb_read() and atomic_mb_set() outside
- a loop. For example, the above definition atomic_mb_read() gives
- the following transformation:
-
- n = 0; n = 0;
- for (i = 0; i < 10; i++) => for (i = 0; i < 10; i++)
- n += atomic_mb_read(&a[i]); n += atomic_read(&a[i]);
- smp_mb_acquire();
-
- Similarly, atomic_mb_set() can be transformed as follows:
-
- smp_mb_release();
- for (i = 0; i < 10; i++) => for (i = 0; i < 10; i++)
- atomic_mb_set(&a[i], false); atomic_set(&a[i], false);
- smp_mb();
-
-
- The other thread can still use atomic_mb_read()/atomic_mb_set().
-
-The two tricks can be combined. In this case, splitting a loop in
-two lets you hoist the barriers out of the loops _and_ eliminate the
-expensive smp_mb():
-
- smp_mb_release();
- for (i = 0; i < 10; i++) { => for (i = 0; i < 10; i++)
- atomic_mb_set(&a[i], false); atomic_set(&a[i], false);
- atomic_mb_set(&b[i], false); smb_wmb();
- } for (i = 0; i < 10; i++)
- atomic_set(&a[i], false);
- smp_mb();
-
-
-Memory barrier pairing
-----------------------
-
-A useful rule of thumb is that memory barriers should always, or almost
-always, be paired with another barrier. In the case of QEMU, however,
-note that the other barrier may actually be in a driver that runs in
-the guest!
-
-For the purposes of pairing, smp_read_barrier_depends() and smp_rmb()
-both count as read barriers. A read barrier shall pair with a write
-barrier or a full barrier; a write barrier shall pair with a read
-barrier or a full barrier. A full barrier can pair with anything.
-For example:
-
- thread 1 thread 2
- =============== ===============
- a = 1;
- smp_wmb();
- b = 2; x = b;
- smp_rmb();
- y = a;
-
-Note that the "writing" thread is accessing the variables in the
-opposite order as the "reading" thread. This is expected: stores
-before the write barrier will normally match the loads after the
-read barrier, and vice versa. The same is true for more than 2
-access and for data dependency barriers:
-
- thread 1 thread 2
- =============== ===============
- b[2] = 1;
- smp_wmb();
- x->i = 2;
- smp_wmb();
- a = x; x = a;
- smp_read_barrier_depends();
- y = x->i;
- smp_read_barrier_depends();
- z = b[y];
-
-smp_wmb() also pairs with atomic_mb_read() and smp_mb_acquire().
-and smp_rmb() also pairs with atomic_mb_set() and smp_mb_release().
-
-
-COMPARISON WITH LINUX KERNEL MEMORY BARRIERS
-============================================
-
-Here is a list of differences between Linux kernel atomic operations
-and memory barriers, and the equivalents in QEMU:
-
-- atomic operations in Linux are always on a 32-bit int type and
- use a boxed atomic_t type; atomic operations in QEMU are polymorphic
- and use normal C types.
-
-- Originally, atomic_read and atomic_set in Linux gave no guarantee
- at all. Linux 4.1 updated them to implement volatile
- semantics via ACCESS_ONCE (or the more recent READ/WRITE_ONCE).
-
- QEMU's atomic_read/set implement, if the compiler supports it, C11
- atomic relaxed semantics, and volatile semantics otherwise.
- Both semantics prevent the compiler from doing certain transformations;
- the difference is that atomic accesses are guaranteed to be atomic,
- while volatile accesses aren't. Thus, in the volatile case we just cross
- our fingers hoping that the compiler will generate atomic accesses,
- since we assume the variables passed are machine-word sized and
- properly aligned.
- No barriers are implied by atomic_read/set in either Linux or QEMU.
-
-- atomic read-modify-write operations in Linux are of three kinds:
-
- atomic_OP returns void
- atomic_OP_return returns new value of the variable
- atomic_fetch_OP returns the old value of the variable
- atomic_cmpxchg returns the old value of the variable
-
- In QEMU, the second kind does not exist. Currently Linux has
- atomic_fetch_or only. QEMU provides and, or, inc, dec, add, sub.
-
-- different atomic read-modify-write operations in Linux imply
- a different set of memory barriers; in QEMU, all of them enforce
- sequential consistency, which means they imply full memory barriers
- before and after the operation.
-
-- Linux does not have an equivalent of atomic_mb_set(). In particular,
- note that smp_store_mb() is a little weaker than atomic_mb_set().
- atomic_mb_read() compiles to the same instructions as Linux's
- smp_load_acquire(), but this should be treated as an implementation
- detail.
-
-SOURCES
-=======
-
-* Documentation/memory-barriers.txt from the Linux kernel
-
-* "The JSR-133 Cookbook for Compiler Writers", available at
- http://g.oswego.edu/dl/jmm/cookbook.html
diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index b734ba4655..a9e1200dff 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -17,6 +17,7 @@ Contents:
loads-stores
memory
migration
+ atomics
stable-process
testing
decodetree
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 4/8] atomics: update documentation
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (2 preceding siblings ...)
2020-04-11 13:03 ` [PULL 3/8] atomics: convert to reStructuredText Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 5/8] rcu: do not mention atomic_mb_read/set in documentation Paolo Bonzini
` (5 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel
Some of the constraints on operand sizes have been relaxed, so adjust the
documentation.
Deprecate atomic_mb_read and atomic_mb_set; it is not really possible to
use them correctly because they do not interoperate with sequentially-consistent
RMW operations.
Finally, extend the memory barrier pairing section to cover acquire and
release semantics in general, roughly based on the KVM Forum 2016 talk,
"<atomic.h> weapons".
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
docs/devel/atomics.rst | 481 +++++++++++++++++++++++------------------
roms/SLOF | 2 +-
2 files changed, 272 insertions(+), 211 deletions(-)
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst
index 83ed3d6981..445c3b3503 100644
--- a/docs/devel/atomics.rst
+++ b/docs/devel/atomics.rst
@@ -11,10 +11,15 @@ that is consistent with the expectations of the programmer.
The most basic tool is locking. Mutexes, condition variables and
semaphores are used in QEMU, and should be the default approach to
synchronization. Anything else is considerably harder, but it's
-also justified more often than one would like. The two tools that
-are provided by ``qemu/atomic.h`` are memory barriers and atomic operations.
+also justified more often than one would like;
+the most performance-critical parts of QEMU in particular require
+a very low level approach to concurrency, involving memory barriers
+and atomic operations. The semantics of concurrent memory accesses are governed
+by the C11 memory model.
-Macros defined by ``qemu/atomic.h`` fall in three camps:
+QEMU provides a header, ``qemu/atomic.h``, which wraps C11 atomics to
+provide better portability and a less verbose syntax. ``qemu/atomic.h``
+provides macros that fall in three camps:
- compiler barriers: ``barrier()``;
@@ -24,13 +29,21 @@ Macros defined by ``qemu/atomic.h`` fall in three camps:
- sequentially consistent atomic access: everything else.
+In general, use of ``qemu/atomic.h`` should be wrapped with more easily
+used data structures (e.g. the lock-free singly-linked list operations
+``QSLIST_INSERT_HEAD_ATOMIC`` and ``QSLIST_MOVE_ATOMIC``) or synchronization
+primitives (such as RCU, ``QemuEvent`` or ``QemuLockCnt``). Bare use of
+atomic operations and memory barriers should be limited to inter-thread
+checking of flags and documented thoroughly.
+
+
Compiler memory barrier
=======================
-``barrier()`` prevents the compiler from moving the memory accesses either
-side of it to the other side. The compiler barrier has no direct effect
-on the CPU, which may then reorder things however it wishes.
+``barrier()`` prevents the compiler from moving the memory accesses on
+either side of it to the other side. The compiler barrier has no direct
+effect on the CPU, which may then reorder things however it wishes.
``barrier()`` is mostly used within ``qemu/atomic.h`` itself. On some
architectures, CPU guarantees are strong enough that blocking compiler
@@ -73,7 +86,8 @@ operations::
typeof(*ptr) atomic_cmpxchg(ptr, old, new)
all of which return the old value of ``*ptr``. These operations are
-polymorphic; they operate on any type that is as wide as a pointer.
+polymorphic; they operate on any type that is as wide as a pointer or
+smaller.
Similar operations return the new value of ``*ptr``::
@@ -85,36 +99,28 @@ Similar operations return the new value of ``*ptr``::
typeof(*ptr) atomic_or_fetch(ptr, val)
typeof(*ptr) atomic_xor_fetch(ptr, val)
-Sequentially consistent loads and stores can be done using::
-
- atomic_fetch_add(ptr, 0) for loads
- atomic_xchg(ptr, val) for stores
-
-However, they are quite expensive on some platforms, notably POWER and
-Arm. Therefore, qemu/atomic.h provides two primitives with slightly
-weaker constraints::
+``qemu/atomic.h`` also provides loads and stores that cannot be reordered
+with each other::
typeof(*ptr) atomic_mb_read(ptr)
void atomic_mb_set(ptr, val)
-The semantics of these primitives map to Java volatile variables,
-and are strongly related to memory barriers as used in the Linux
-kernel (see below).
+However these do not provide sequential consistency and, in particular,
+they do not participate in the total ordering enforced by
+sequentially-consistent operations. For this reason they are deprecated.
+They should instead be replaced with any of the following (ordered from
+easiest to hardest):
-As long as you use atomic_mb_read and atomic_mb_set, accesses cannot
-be reordered with each other, and it is also not possible to reorder
-"normal" accesses around them.
+- accesses inside a mutex or spinlock
-However, and this is the important difference between
-atomic_mb_read/atomic_mb_set and sequential consistency, it is important
-for both threads to access the same volatile variable. It is not the
-case that everything visible to thread A when it writes volatile field f
-becomes visible to thread B after it reads volatile field g. The store
-and load have to "match" (i.e., be performed on the same volatile
-field) to achieve the right semantics.
+- lightweight synchronization primitives such as ``QemuEvent``
+- RCU operations (``atomic_rcu_read``, ``atomic_rcu_set``) when publishing
+ or accessing a new version of a data structure
-These operations operate on any type that is as wide as an int or smaller.
+- other atomic accesses: ``atomic_read`` and ``atomic_load_acquire`` for
+ loads, ``atomic_set`` and ``atomic_store_release`` for stores, ``smp_mb``
+ to forbid reordering subsequent loads before a store.
Weak atomic access and manual memory barriers
@@ -122,9 +128,24 @@ Weak atomic access and manual memory barriers
Compared to sequentially consistent atomic access, programming with
weaker consistency models can be considerably more complicated.
-In general, if the algorithm you are writing includes both writes
-and reads on the same side, it is generally simpler to use sequentially
-consistent primitives.
+The only guarantees that you can rely upon in this case are:
+
+- atomic accesses will not cause data races (and hence undefined behavior);
+ ordinary accesses instead cause data races if they are concurrent with
+ other accesses of which at least one is a write. In order to ensure this,
+ the compiler will not optimize accesses out of existence, create unsolicited
+ accesses, or perform other similar optimzations.
+
+- acquire operations will appear to happen, with respect to the other
+ components of the system, before all the LOAD or STORE operations
+ specified afterwards.
+
+- release operations will appear to happen, with respect to the other
+ components of the system, after all the LOAD or STORE operations
+ specified before.
+
+- release operations will *synchronize with* acquire operations;
+ see :ref:`acqrel` for a detailed explanation.
When using this model, variables are accessed with:
@@ -142,9 +163,9 @@ When using this model, variables are accessed with:
- ``atomic_store_release()``, which guarantees the STORE to appear to
happen, with respect to the other components of the system,
- after all the LOAD or STORE operations specified afterwards.
+ after all the LOAD or STORE operations specified before.
Operations coming after ``atomic_store_release()`` can still be
- reordered after it.
+ reordered before it.
Restrictions to the ordering of accesses can also be specified
using the memory barrier macros: ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
@@ -208,168 +229,188 @@ They come in six kinds:
dependency and a full read barrier or better is required.
-This is the set of barriers that is required *between* two ``atomic_read()``
-and ``atomic_set()`` operations to achieve sequential consistency:
-
- +----------------+-------------------------------------------------------+
- | | 2nd operation |
- | +------------------+-----------------+------------------+
- | 1st operation | (after last) | atomic_read | atomic_set |
- +----------------+------------------+-----------------+------------------+
- | (before first) | .. | none | smp_mb_release() |
- +----------------+------------------+-----------------+------------------+
- | atomic_read | smp_mb_acquire() | smp_rmb() [1]_ | [2]_ |
- +----------------+------------------+-----------------+------------------+
- | atomic_set | none | smp_mb() [3]_ | smp_wmb() |
- +----------------+------------------+-----------------+------------------+
-
- .. [1] Or smp_read_barrier_depends().
-
- .. [2] This requires a load-store barrier. This is achieved by
- either smp_mb_acquire() or smp_mb_release().
-
- .. [3] This requires a store-load barrier. On most machines, the only
- way to achieve this is a full barrier.
-
-
-You can see that the two possible definitions of ``atomic_mb_read()``
-and ``atomic_mb_set()`` are the following:
-
- 1) | atomic_mb_read(p) = atomic_read(p); smp_mb_acquire()
- | atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v); smp_mb()
-
- 2) | atomic_mb_read(p) = smp_mb() atomic_read(p); smp_mb_acquire()
- | atomic_mb_set(p, v) = smp_mb_release(); atomic_set(p, v);
-
-Usually the former is used, because ``smp_mb()`` is expensive and a program
-normally has more reads than writes. Therefore it makes more sense to
-make ``atomic_mb_set()`` the more expensive operation.
-
-There are two common cases in which atomic_mb_read and atomic_mb_set
-generate too many memory barriers, and thus it can be useful to manually
-place barriers, or use atomic_load_acquire/atomic_store_release instead:
-
-- when a data structure has one thread that is always a writer
- and one thread that is always a reader, manual placement of
- memory barriers makes the write side faster. Furthermore,
- correctness is easy to check for in this case using the "pairing"
- trick that is explained below:
-
- +----------------------------------------------------------------------+
- | thread 1 |
- +-----------------------------------+----------------------------------+
- | before | after |
- +===================================+==================================+
- | :: | :: |
- | | |
- | (other writes) | |
- | atomic_mb_set(&a, x) | atomic_store_release(&a, x) |
- | atomic_mb_set(&b, y) | atomic_store_release(&b, y) |
- +-----------------------------------+----------------------------------+
-
- +----------------------------------------------------------------------+
- | thread 2 |
- +-----------------------------------+----------------------------------+
- | before | after |
- +===================================+==================================+
- | :: | :: |
- | | |
- | y = atomic_mb_read(&b) | y = atomic_load_acquire(&b) |
- | x = atomic_mb_read(&a) | x = atomic_load_acquire(&a) |
- | (other reads) | |
- +-----------------------------------+----------------------------------+
-
- Note that the barrier between the stores in thread 1, and between
- the loads in thread 2, has been optimized here to a write or a
- read memory barrier respectively. On some architectures, notably
- ARMv7, smp_mb_acquire and smp_mb_release are just as expensive as
- smp_mb, but smp_rmb and/or smp_wmb are more efficient.
-
-- sometimes, a thread is accessing many variables that are otherwise
- unrelated to each other (for example because, apart from the current
- thread, exactly one other thread will read or write each of these
- variables). In this case, it is possible to "hoist" the implicit
- barriers provided by ``atomic_mb_read()`` and ``atomic_mb_set()`` outside
- a loop. For example, the above definition ``atomic_mb_read()`` gives
- the following transformation:
-
- +-----------------------------------+----------------------------------+
- | before | after |
- +===================================+==================================+
- | :: | :: |
- | | |
- | n = 0; | n = 0; |
- | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
- | n += atomic_mb_read(&a[i]); | n += atomic_read(&a[i]); |
- | | smp_mb_acquire(); |
- +-----------------------------------+----------------------------------+
-
- Similarly, atomic_mb_set() can be transformed as follows:
-
- +-----------------------------------+----------------------------------+
- | before | after |
- +===================================+==================================+
- | :: | :: |
- | | |
- | | smp_mb_release(); |
- | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
- | atomic_mb_set(&a[i], false); | atomic_set(&a[i], false); |
- | | smp_mb(); |
- +-----------------------------------+----------------------------------+
-
-
- The other thread can still use ``atomic_mb_read()``/``atomic_mb_set()``.
-
-The two tricks can be combined. In this case, splitting a loop in
-two lets you hoist the barriers out of the loops _and_ eliminate the
-expensive ``smp_mb()``:
-
- +-----------------------------------+----------------------------------+
- | before | after |
- +===================================+==================================+
- | :: | :: |
- | | |
- | | smp_mb_release(); |
- | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
- | atomic_mb_set(&a[i], false); | atomic_set(&a[i], false); |
- | atomic_mb_set(&b[i], false); | smb_wmb(); |
- | } | for (i = 0; i < 10; i++) |
- | | atomic_set(&a[i], false); |
- | | smp_mb(); |
- +-----------------------------------+----------------------------------+
-
-
-Memory barrier pairing
-----------------------
-
-A useful rule of thumb is that memory barriers should always, or almost
-always, be paired with another barrier. In the case of QEMU, however,
-note that the other barrier may actually be in a driver that runs in
-the guest!
-
-For the purposes of pairing, ``smp_read_barrier_depends()`` and ``smp_rmb()``
-both count as read barriers. A read barrier shall pair with a write
-barrier or a full barrier; a write barrier shall pair with a read
-barrier or a full barrier. A full barrier can pair with anything.
-For example:
-
- +--------------------+------------------------------+
- | thread 1 | thread 2 |
- +====================+==============================+
- | :: | :: |
- | | |
- | a = 1; | |
- | smp_wmb(); | |
- | b = 2; | x = b; |
- | | smp_rmb(); |
- | | y = a; |
- +--------------------+------------------------------+
+Memory barriers and ``atomic_load_acquire``/``atomic_store_release`` are
+mostly used when a data structure has one thread that is always a writer
+and one thread that is always a reader:
+
+ +----------------------------------+----------------------------------+
+ | thread 1 | thread 2 |
+ +==================================+==================================+
+ | :: | :: |
+ | | |
+ | atomic_store_release(&a, x); | y = atomic_load_acquire(&b); |
+ | atomic_store_release(&b, y); | x = atomic_load_acquire(&a); |
+ +----------------------------------+----------------------------------+
+
+In this case, correctness is easy to check for using the "pairing"
+trick that is explained below.
+
+Sometimes, a thread is accessing many variables that are otherwise
+unrelated to each other (for example because, apart from the current
+thread, exactly one other thread will read or write each of these
+variables). In this case, it is possible to "hoist" the barriers
+outside a loop. For example:
+
+ +------------------------------------------+----------------------------------+
+ | before | after |
+ +==========================================+==================================+
+ | :: | :: |
+ | | |
+ | n = 0; | n = 0; |
+ | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
+ | n += atomic_load_acquire(&a[i]); | n += atomic_read(&a[i]); |
+ | | smp_mb_acquire(); |
+ +------------------------------------------+----------------------------------+
+ | :: | :: |
+ | | |
+ | | smp_mb_release(); |
+ | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
+ | atomic_store_release(&a[i], false); | atomic_set(&a[i], false); |
+ +------------------------------------------+----------------------------------+
+
+Splitting a loop can also be useful to reduce the number of barriers:
+
+ +------------------------------------------+----------------------------------+
+ | before | after |
+ +==========================================+==================================+
+ | :: | :: |
+ | | |
+ | n = 0; | smp_mb_release(); |
+ | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
+ | atomic_store_release(&a[i], false); | atomic_set(&a[i], false); |
+ | smp_mb(); | smb_mb(); |
+ | n += atomic_read(&b[i]); | n = 0; |
+ | } | for (i = 0; i < 10; i++) |
+ | | n += atomic_read(&b[i]); |
+ +------------------------------------------+----------------------------------+
+
+In this case, a ``smp_mb_release()`` is also replaced with a (possibly cheaper, and clearer
+as well) ``smp_wmb()``:
+
+ +------------------------------------------+----------------------------------+
+ | before | after |
+ +==========================================+==================================+
+ | :: | :: |
+ | | |
+ | | smp_mb_release(); |
+ | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
+ | atomic_store_release(&a[i], false); | atomic_set(&a[i], false); |
+ | atomic_store_release(&b[i], false); | smb_wmb(); |
+ | } | for (i = 0; i < 10; i++) |
+ | | atomic_set(&b[i], false); |
+ +------------------------------------------+----------------------------------+
+
+
+.. _acqrel:
+
+Acquire/release pairing and the *synchronizes-with* relation
+------------------------------------------------------------
+
+Atomic operations other than ``atomic_set()`` and ``atomic_read()`` have
+either *acquire* or *release* semantics [#rmw]_. This has two effects:
+
+.. [#rmw] Read-modify-write operations can have both---acquire applies to the
+ read part, and release to the write.
+
+- within a thread, they are ordered either before subsequent operations
+ (for acquire) or after previous operations (for release).
+
+- if a release operation in one thread *synchronizes with* an acquire operation
+ in another thread, the ordering constraints propagates from the first to the
+ second thread. That is, everything before the release operation in the
+ first thread is guaranteed to *happen before* everything after the
+ acquire operation in the second thread.
+
+The concept of acquire and release semantics is not exclusive to atomic
+operations; almost all higher-level synchronization primitives also have
+acquire or release semantics. For example:
+
+- ``pthread_mutex_lock`` has acquire semantics, ``pthread_mutex_unlock`` has
+ release semantics and synchronizes with a ``pthread_mutex_lock`` for the
+ same mutex.
+
+- ``pthread_cond_signal`` and ``pthread_cond_broadcast`` have release semantics;
+ ``pthread_cond_wait`` has both release semantics (synchronizing with
+ ``pthread_mutex_lock``) and acquire semantics (synchronizing with
+ ``pthread_mutex_unlock`` and signaling of the condition variable).
+
+- ``pthread_create`` has release semantics and synchronizes with the start
+ of the new thread; ``pthread_join`` has acquire semantics and synchronizes
+ with the exiting of the thread.
+
+- ``qemu_event_set`` has release semantics, ``qemu_event_wait`` has
+ acquire semantics.
+
+For example, in the following example there are no atomic accesses, but still
+thread 2 is relying on the *synchronizes-with* relation between ``pthread_exit``
+(release) and ``pthread_join`` (acquire):
+
+ +----------------------+-------------------------------+
+ | thread 1 | thread 2 |
+ +======================+===============================+
+ | :: | :: |
+ | | |
+ | *a = 1; | |
+ | pthread_exit(a); | pthread_join(thread1, &a); |
+ | | x = *a; |
+ +----------------------+-------------------------------+
+
+Synchronization between threads basically descends from this pairing of
+a release operation and an acquire operation. Therefore, atomic operations
+other than ``atomic_set()`` and ``atomic_read()`` will almost always be
+paired with another operation of the opposite kind: an acquire operation
+will pair with a release operation and vice versa. This rule of thumb is
+extremely useful; in the case of QEMU, however, note that the other
+operation may actually be in a driver that runs in the guest!
+
+``smp_read_barrier_depends()``, ``smp_rmb()``, ``smp_mb_acquire()``,
+``atomic_load_acquire()`` and ``atomic_rcu_read()`` all count
+as acquire operations. ``smp_wmb()``, ``smp_mb_release()``,
+``atomic_store_release()`` and ``atomic_rcu_set()`` all count as release
+operations. ``smp_mb()`` counts as both acquire and release, therefore
+it can pair with any other atomic operation. Here is an example:
+
+ +----------------------+------------------------------+
+ | thread 1 | thread 2 |
+ +======================+==============================+
+ | :: | :: |
+ | | |
+ | atomic_set(&a, 1); | |
+ | smp_wmb(); | |
+ | atomic_set(&b, 2); | x = atomic_read(&b); |
+ | | smp_rmb(); |
+ | | y = atomic_read(&a); |
+ +----------------------+------------------------------+
+
+Note that a load-store pair only counts if the two operations access the
+same variable: that is, a store-release on a variable ``x`` *synchronizes
+with* a load-acquire on a variable ``x``, while a release barrier
+synchronizes with any acquire operation. The following example shows
+correct synchronization:
+
+ +--------------------------------+--------------------------------+
+ | thread 1 | thread 2 |
+ +================================+================================+
+ | :: | :: |
+ | | |
+ | atomic_set(&a, 1); | |
+ | atomic_store_release(&b, 2); | x = atomic_load_acquire(&b); |
+ | | y = atomic_read(&a); |
+ +--------------------------------+--------------------------------+
+
+Acquire and release semantics of higher-level primitives can also be
+relied upon for the purpose of establishing the *synchronizes with*
+relation.
Note that the "writing" thread is accessing the variables in the
opposite order as the "reading" thread. This is expected: stores
-before the write barrier will normally match the loads after the
-read barrier, and vice versa. The same is true for more than 2
-access and for data dependency barriers:
+before a release operation will normally match the loads after
+the acquire operation, and vice versa. In fact, this happened already
+in the ``pthread_exit``/``pthread_join`` example above.
+
+Finally, this more complex example has more than two accesses and data
+dependency barriers. It also does not use atomic accesses whenever there
+cannot be a data race:
+----------------------+------------------------------+
| thread 1 | thread 2 |
@@ -380,19 +421,15 @@ access and for data dependency barriers:
| smp_wmb(); | |
| x->i = 2; | |
| smp_wmb(); | |
- | a = x; | x = a; |
+ | atomic_set(&a, x); | x = atomic_read(&a); |
| | smp_read_barrier_depends(); |
| | y = x->i; |
| | smp_read_barrier_depends(); |
| | z = b[y]; |
+----------------------+------------------------------+
-``smp_wmb()`` also pairs with ``atomic_mb_read()`` and ``smp_mb_acquire()``.
-and ``smp_rmb()`` also pairs with ``atomic_mb_set()`` and ``smp_mb_release()``.
-
-
-Comparison with Linux kernel memory barriers
-============================================
+Comparison with Linux kernel primitives
+=======================================
Here is a list of differences between Linux kernel atomic operations
and memory barriers, and the equivalents in QEMU:
@@ -426,19 +463,43 @@ and memory barriers, and the equivalents in QEMU:
``atomic_cmpxchg`` returns the old value of the variable
===================== =========================================
- In QEMU, the second kind does not exist. Currently Linux has
- atomic_fetch_or only. QEMU provides and, or, inc, dec, add, sub.
+ In QEMU, the second kind is named ``atomic_OP_fetch``.
- different atomic read-modify-write operations in Linux imply
a different set of memory barriers; in QEMU, all of them enforce
- sequential consistency, which means they imply full memory barriers
- before and after the operation.
-
-- Linux does not have an equivalent of ``atomic_mb_set()``. In particular,
- note that ``smp_store_mb()`` is a little weaker than ``atomic_mb_set()``.
- ``atomic_mb_read()`` compiles to the same instructions as Linux's
- ``smp_load_acquire()``, but this should be treated as an implementation
- detail.
+ sequential consistency.
+
+- in QEMU, ``atomic_read()`` and ``atomic_set()`` do not participate in
+ the total ordering enforced by sequentially-consistent operations.
+ This is because QEMU uses the C11 memory model. The following example
+ is correct in Linux but not in QEMU:
+
+ +----------------------------------+--------------------------------+
+ | Linux (correct) | QEMU (incorrect) |
+ +==================================+================================+
+ | :: | :: |
+ | | |
+ | a = atomic_fetch_add(&x, 2); | a = atomic_fetch_add(&x, 2); |
+ | b = READ_ONCE(&y); | b = atomic_read(&y); |
+ +----------------------------------+--------------------------------+
+
+ because the read of ``y`` can be moved (by either the processor or the
+ compiler) before the write of ``x``.
+
+ Fixing this requires an ``smp_mb()`` memory barrier between the write
+ of ``x`` and the read of ``y``. In the common case where only one thread
+ writes ``x``, it is also possible to write it like this:
+
+ +--------------------------------+
+ | QEMU (correct) |
+ +================================+
+ | :: |
+ | |
+ | a = atomic_read(&x); |
+ | atomic_set(&x, a + 2); |
+ | smp_mb(); |
+ | b = atomic_read(&y); |
+ +--------------------------------+
Sources
=======
diff --git a/roms/SLOF b/roms/SLOF
index ab6984f5a6..9546892a80 160000
--- a/roms/SLOF
+++ b/roms/SLOF
@@ -1 +1 @@
-Subproject commit ab6984f5a6d054e1f634dda855b32e5357111974
+Subproject commit 9546892a80d5a4c73deea6719de46372f007f4a6
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 5/8] rcu: do not mention atomic_mb_read/set in documentation
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (3 preceding siblings ...)
2020-04-11 13:03 ` [PULL 4/8] atomics: update documentation Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 6/8] vl.c: error out if -mem-path is used together with -M memory-backend Paolo Bonzini
` (4 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
docs/devel/rcu.txt | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/docs/devel/rcu.txt b/docs/devel/rcu.txt
index d83fed2f79..0ce15ba198 100644
--- a/docs/devel/rcu.txt
+++ b/docs/devel/rcu.txt
@@ -132,7 +132,7 @@ The core RCU API is small:
typeof(*p) atomic_rcu_read(p);
- atomic_rcu_read() is similar to atomic_mb_read(), but it makes
+ atomic_rcu_read() is similar to atomic_load_acquire(), but it makes
some assumptions on the code that calls it. This allows a more
optimized implementation.
@@ -154,7 +154,7 @@ The core RCU API is small:
void atomic_rcu_set(p, typeof(*p) v);
- atomic_rcu_set() is also similar to atomic_mb_set(), and it also
+ atomic_rcu_set() is similar to atomic_store_release(), though it also
makes assumptions on the code that calls it in order to allow a more
optimized implementation.
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 6/8] vl.c: error out if -mem-path is used together with -M memory-backend
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (4 preceding siblings ...)
2020-04-11 13:03 ` [PULL 5/8] rcu: do not mention atomic_mb_read/set in documentation Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 7/8] memory: Do not allow direct write access to rom_device regions Paolo Bonzini
` (3 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Igor Mammedov
From: Igor Mammedov <imammedo@redhat.com>
the former is not actually used by explicit backend, so instead of
silently ignoring the option in non valid context, exit with error.
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200409134133.11339-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
softmmu/vl.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/softmmu/vl.c b/softmmu/vl.c
index 58a40bcfc1..32c0047889 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -4315,6 +4315,11 @@ void qemu_init(int argc, char **argv, char **envp)
"explicitly specified 'memory-backend' property");
exit(EXIT_FAILURE);
}
+ if (mem_path) {
+ error_report("'-mem-path' can't be used together with"
+ "'-machine memory-backend'");
+ exit(EXIT_FAILURE);
+ }
ram_size = backend_size;
}
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 7/8] memory: Do not allow direct write access to rom_device regions
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (5 preceding siblings ...)
2020-04-11 13:03 ` [PULL 6/8] vl.c: error out if -mem-path is used together with -M memory-backend Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 13:03 ` [PULL 8/8] module: increase dirs array size by one Paolo Bonzini
` (2 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Alexander Duyck
From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
According to the documentation in memory.h a ROM memory region will be
backed by RAM for reads, but is supposed to go through a callback for
writes. Currently we were not checking for the existence of the rom_device
flag when determining if we could perform a direct write or not.
To correct that add a check to memory_region_is_direct so that if the
memory region has the rom_device flag set we will return false for all
checks where is_write is set.
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-Id: <20200410034150.24738.98143.stgit@localhost.localdomain>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
include/exec/memory.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 1614d9a02c..e000bd2f97 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -2351,8 +2351,8 @@ void address_space_write_cached_slow(MemoryRegionCache *cache,
static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
{
if (is_write) {
- return memory_region_is_ram(mr) &&
- !mr->readonly && !memory_region_is_ram_device(mr);
+ return memory_region_is_ram(mr) && !mr->readonly &&
+ !mr->rom_device && !memory_region_is_ram_device(mr);
} else {
return (memory_region_is_ram(mr) && !memory_region_is_ram_device(mr)) ||
memory_region_is_romd(mr);
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PULL 8/8] module: increase dirs array size by one
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (6 preceding siblings ...)
2020-04-11 13:03 ` [PULL 7/8] memory: Do not allow direct write access to rom_device regions Paolo Bonzini
@ 2020-04-11 13:03 ` Paolo Bonzini
2020-04-11 16:54 ` [PULL 0/8] Misc patches for QEMU 5.0-rc3 Peter Maydell
2020-04-14 7:36 ` Igor Mammedov
9 siblings, 0 replies; 11+ messages in thread
From: Paolo Bonzini @ 2020-04-11 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Bruce Rogers
From: Bruce Rogers <brogers@suse.com>
With the module upgrades code change, the statically sized dirs array
can now overflow. Increase it's size by one, according to the new
maximum possible usage.
Fixes: bd83c861c0 ("modules: load modules from versioned /var/run dir")
Signed-off-by: Bruce Rogers <brogers@suse.com>
Message-Id: <20200411010746.472295-1-brogers@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
util/module.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/util/module.c b/util/module.c
index 5f7896870a..e48d9aacc0 100644
--- a/util/module.c
+++ b/util/module.c
@@ -177,7 +177,7 @@ bool module_load_one(const char *prefix, const char *lib_name)
char *version_dir;
#endif
const char *search_dir;
- char *dirs[4];
+ char *dirs[5];
char *module_name;
int i = 0, n_dirs = 0;
int ret;
--
2.18.2
^ permalink raw reply related [flat|nested] 11+ messages in thread
* Re: [PULL 0/8] Misc patches for QEMU 5.0-rc3
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (7 preceding siblings ...)
2020-04-11 13:03 ` [PULL 8/8] module: increase dirs array size by one Paolo Bonzini
@ 2020-04-11 16:54 ` Peter Maydell
2020-04-14 7:36 ` Igor Mammedov
9 siblings, 0 replies; 11+ messages in thread
From: Peter Maydell @ 2020-04-11 16:54 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: QEMU Developers
On Sat, 11 Apr 2020 at 14:04, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The following changes since commit 53ef8a92eb04ee19640f5aad3bff36cd4a36c250:
>
> Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20200406' into staging (2020-04-06 12:36:45 +0100)
>
> are available in the Git repository at:
>
> git://github.com/bonzini/qemu.git tags/for-upstream
>
> for you to fetch changes up to 8c12bd8e32faf8c483cd96c1557899120bc67eea:
>
> module: increase dirs array size by one (2020-04-11 08:49:26 -0400)
>
> ----------------------------------------------------------------
> Bugfixes, and reworking of the atomics documentation.
>
>
> docs/devel/atomics.rst | 507 +++++++++++++++++++++++++++++++++++
> docs/devel/atomics.txt | 403 ----------------------------
> docs/devel/index.rst | 1 +
> docs/devel/rcu.txt | 4 +-
> hw/i386/pc_piix.c | 19 +-
> include/exec/memory.h | 4 +-
> roms/SLOF | 2 +-
It looks like this roms/SLOF change is bogus (it's in your
"atomics: update documentation" commit) and it makes git
complain about the merge attempt:
Failed to merge submodule roms/SLOF (commits don't follow merge-base)
Auto-merging roms/SLOF
CONFLICT (submodule): Merge conflict in roms/SLOF
Removing docs/devel/atomics.txt
Automatic merge failed; fix conflicts and then commit the result.
thanks
-- PMM
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PULL 0/8] Misc patches for QEMU 5.0-rc3
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
` (8 preceding siblings ...)
2020-04-11 16:54 ` [PULL 0/8] Misc patches for QEMU 5.0-rc3 Peter Maydell
@ 2020-04-14 7:36 ` Igor Mammedov
9 siblings, 0 replies; 11+ messages in thread
From: Igor Mammedov @ 2020-04-14 7:36 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: qemu-devel
On Sat, 11 Apr 2020 09:03:22 -0400
Paolo Bonzini <pbonzini@redhat.com> wrote:
> The following changes since commit 53ef8a92eb04ee19640f5aad3bff36cd4a36c250:
>
> Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20200406' into staging (2020-04-06 12:36:45 +0100)
>
> are available in the Git repository at:
>
> git://github.com/bonzini/qemu.git tags/for-upstream
>
> for you to fetch changes up to 8c12bd8e32faf8c483cd96c1557899120bc67eea:
>
> module: increase dirs array size by one (2020-04-11 08:49:26 -0400)
>
> ----------------------------------------------------------------
> Bugfixes, and reworking of the atomics documentation.
>
> ----------------------------------------------------------------
> Alexander Duyck (1):
> memory: Do not allow direct write access to rom_device regions
>
> Bauerchen (1):
> oslib-posix: take lock before qemu_cond_broadcast
>
> Bruce Rogers (1):
> module: increase dirs array size by one
>
> Igor Mammedov (1):
> vl.c: error out if -mem-path is used together with -M memory-backend
It seems one more patch was forgotten
"[PATCH for-5.0 0/2] fix qemu crash with memory-backend-* prealloc=yes"
>
> Olaf Hering (1):
> piix: fix xenfv regression, add compat machine xenfv-4.2
>
> Paolo Bonzini (3):
> atomics: convert to reStructuredText
> atomics: update documentation
> rcu: do not mention atomic_mb_read/set in documentation
>
> docs/devel/atomics.rst | 507 +++++++++++++++++++++++++++++++++++
> docs/devel/atomics.txt | 403 ----------------------------
> docs/devel/index.rst | 1 +
> docs/devel/rcu.txt | 4 +-
> hw/i386/pc_piix.c | 19 +-
> include/exec/memory.h | 4 +-
> roms/SLOF | 2 +-
> softmmu/vl.c | 5 +
> tests/qtest/device-introspect-test.c | 2 +-
> tests/qtest/qom-test.c | 42 +--
> tests/qtest/test-hmp.c | 2 +-
> util/module.c | 2 +-
> util/oslib-posix.c | 3 +
> 13 files changed, 548 insertions(+), 448 deletions(-)
> create mode 100644 docs/devel/atomics.rst
> delete mode 100644 docs/devel/atomics.txt
^ permalink raw reply [flat|nested] 11+ messages in thread
end of thread, other threads:[~2020-04-14 7:38 UTC | newest]
Thread overview: 11+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-04-11 13:03 [PULL 0/8] Misc patches for QEMU 5.0-rc3 Paolo Bonzini
2020-04-11 13:03 ` [PULL 1/8] piix: fix xenfv regression, add compat machine xenfv-4.2 Paolo Bonzini
2020-04-11 13:03 ` [PULL 2/8] oslib-posix: take lock before qemu_cond_broadcast Paolo Bonzini
2020-04-11 13:03 ` [PULL 3/8] atomics: convert to reStructuredText Paolo Bonzini
2020-04-11 13:03 ` [PULL 4/8] atomics: update documentation Paolo Bonzini
2020-04-11 13:03 ` [PULL 5/8] rcu: do not mention atomic_mb_read/set in documentation Paolo Bonzini
2020-04-11 13:03 ` [PULL 6/8] vl.c: error out if -mem-path is used together with -M memory-backend Paolo Bonzini
2020-04-11 13:03 ` [PULL 7/8] memory: Do not allow direct write access to rom_device regions Paolo Bonzini
2020-04-11 13:03 ` [PULL 8/8] module: increase dirs array size by one Paolo Bonzini
2020-04-11 16:54 ` [PULL 0/8] Misc patches for QEMU 5.0-rc3 Peter Maydell
2020-04-14 7:36 ` Igor Mammedov
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