[Qemu-devel] [PATCH v4 0/1] Exploit settable KVM_CAP_PPC_SMT

[Qemu-devel] [PATCH v4 0/1] Exploit settable KVM_CAP_PPC_SMT

[Sam Bobroff]

Hello QEMU PPC people,

This is v3, it is only a single patch now.

My core objective with this patch is to provide a way for QEMU to configure the
newly writeable KVM capability 'KVM_CAP_PPC_SMT', because without it Power 9
hosts can only run VMs with a single thread per core. (With this capability they
are able to run VMs with 1, 2, 4 or 8 threads per core.) KVM also now contains a
new read-only property ('KVM_CAP_PPC_SMT_POSSIBLE') to expose the possible
valid values of KVM_CAP_PPC_SMT, although this is only used in a hint to the
user at this stage. This new capability is already upstream and the QEMU
headers have already been updated to include it.

The new way KVM_CAP_PPC_SMT works is that, when set, it causes KVM to act as if
the host's native number of threads per core were the value of the capability.

I've implemented this by adding a new property to pseries machines, which is
stored in sPAPRMachineState as 'vsmt'. This provides a way to set it from the
command line (and a way to add it to the VMState if we later decide to do so)
and makes the property available only when using SPAPR (pseries) machines. I
use this value to call in to KVM and set the capability when necessary.

For pseries machines, the vsmt value will be a duplicate of the KVM capability
value, and in version 1 I tried to remove this duplication by replacing
cap_ppc_smt completely with references to spapr->vsmt. Unfortunately, that
forced generic code (e.g. translate_init.c) to have knowledge of the
sPAPRMachine state which didn't seem conceptually clean. In version 2, I've
left the capability on the KVM side which keeps the KVM and generic code clear
of SPAPR concepts. I think on the whole this is a better solution.

Notes/Questions:
* I've moved the code that validates smp_threads out of ppc_cpu_realizefn()
  because it only needs to be done once, not once per CPU.

Patch set changelog follows:

====== Version 3 -> version 4: ======

Patch 1/1: PPC: KVM: Support machine option to set VSMT mode
* Use "pseries" as the machine name in some hint messages rather than the
  specific instance name, as that's sufficient.
* Merged show_vsmt_possible() and kvmppc_smt_possible() into a single function
  and moved it into kvm.c.
* Altered spapr_set_vsmt_mode() to take an error context. Note that this had
  some follow-on effects:
    * A local error context had to be added because I'm adding hints to the
      error.
    * show_vsmt_possible() now appends an error hint.


Sam Bobroff (1):
  PPC: KVM: Support machine option to set VSMT mode

 hw/ppc/spapr.c              | 75 +++++++++++++++++++++++++++++++++++++++++++++
 include/hw/ppc/spapr.h      |  1 +
 target/ppc/kvm.c            | 39 ++++++++++++++++++++++-
 target/ppc/kvm_ppc.h        | 12 ++++++++
 target/ppc/translate_init.c | 14 ---------
 5 files changed, 126 insertions(+), 15 deletions(-)

-- 
2.14.1.2.g4274c698f

[Qemu-devel] [PATCH v4 1/1] PPC: KVM: Support machine option to set VSMT mode

[Sam Bobroff]

KVM now allows writing to KVM_CAP_PPC_SMT which has previously been
read only. Doing so causes KVM to act, for that VM, as if the host's
SMT mode was the given value. This is particularly important on Power
9 systems because their default value is 1, but they are able to
support values up to 8.

This patch introduces a way to control this capability via a new
machine property called VSMT ("Virtual SMT"). If the value is not set
on the command line a default is chosen that is, when possible,
compatible with legacy systems.

Note that the intialization of KVM_CAP_PPC_SMT has changed slightly
because it has changed (in KVM) from a global capability to a
VM-specific one. This won't cause a problem on older KVMs because VM
capabilities fall back to global ones.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
---
====== Version 3 -> version 4: ======

* Use "pseries" as the machine name in some hint messages rather than the
  specific instance name, as that's sufficient.
* Merged show_vsmt_possible() and kvmppc_smt_possible() into a single function
  and moved it into kvm.c.
* Altered spapr_set_vsmt_mode() to take an error context. Note that this had
  some follow-on effects:
    * A local error context had to be added because I'm adding hints to the
      error.
    * show_vsmt_possible() now appends an error hint.

 hw/ppc/spapr.c              | 75 +++++++++++++++++++++++++++++++++++++++++++++
 include/hw/ppc/spapr.h      |  1 +
 target/ppc/kvm.c            | 39 ++++++++++++++++++++++-
 target/ppc/kvm_ppc.h        | 12 ++++++++
 target/ppc/translate_init.c | 14 ---------
 5 files changed, 126 insertions(+), 15 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index cd6eb2d4a9..7859367154 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -26,6 +26,7 @@
  */
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qapi/visitor.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/numa.h"
 #include "hw/hw.h"
@@ -2140,6 +2141,61 @@ static void spapr_init_cpus(sPAPRMachineState *spapr)
     g_free(type);
 }
 
+static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
+{
+    Error *local_err = NULL;
+    bool vsmt_user = !!spapr->vsmt;
+    int kvm_smt = kvmppc_smt_threads();
+    int ret;
+
+    if (!kvm_enabled() && (smp_threads > 1)) {
+        error_setg(&local_err, "TCG cannot support more than 1 thread/core "
+                     "on a pseries machine");
+        goto out;
+    }
+    if (!is_power_of_2(smp_threads)) {
+        error_setg(&local_err, "Cannot support %d threads/core on a pseries "
+                     "machine because it must be a power of 2", smp_threads);
+        goto out;
+    }
+
+    /* Detemine the VSMT mode to use: */
+    if (vsmt_user) {
+        if (spapr->vsmt < smp_threads) {
+            error_setg(&local_err, "Cannot support VSMT mode %d"
+                         " because it must be >= threads/core (%d)",
+                         spapr->vsmt, smp_threads);
+            goto out;
+        }
+        /* In this case, spapr->vsmt has been set by the command line */
+    } else {
+        /* Choose a VSMT mode that may be higher than necessary but is
+         * likely to be compatible with hosts that don't have VSMT. */
+        spapr->vsmt = MAX(kvm_smt, smp_threads);
+    }
+
+    /* KVM: If necessary, set the SMT mode: */
+    if (kvm_enabled() && (spapr->vsmt != kvm_smt)) {
+        ret = kvmppc_set_smt_threads(spapr->vsmt);
+        if (ret) {
+            error_setg(&local_err,
+                       "Failed to set KVM's VSMT mode to %d (errno %d)",
+                       spapr->vsmt, ret);
+            if (!vsmt_user) {
+                error_append_hint(&local_err, "On PPC, a VM with %d threads/"
+                             "core on a host with %d threads/core requires "
+                             " the use of VSMT mode %d.\n",
+                             smp_threads, kvm_smt, spapr->vsmt);
+            }
+            kvmppc_hint_smt_possible(&local_err);
+            goto out;
+        }
+    }
+    /* else TCG: nothing to do currently */
+out:
+    error_propagate(errp, local_err);
+}
+
 /* pSeries LPAR / sPAPR hardware init */
 static void ppc_spapr_init(MachineState *machine)
 {
@@ -2272,6 +2328,8 @@ static void ppc_spapr_init(MachineState *machine)
 
     spapr_cpu_parse_features(spapr);
 
+    spapr_set_vsmt_mode(spapr, &error_fatal);
+
     spapr_init_cpus(spapr);
 
     if (kvm_enabled()) {
@@ -2656,6 +2714,18 @@ static void spapr_set_resize_hpt(Object *obj, const char *value, Error **errp)
     }
 }
 
+static void spapr_get_vsmt(Object *obj, Visitor *v, const char *name,
+                                   void *opaque, Error **errp)
+{
+    visit_type_uint32(v, name, (uint32_t *)opaque, errp);
+}
+
+static void spapr_set_vsmt(Object *obj, Visitor *v, const char *name,
+                                   void *opaque, Error **errp)
+{
+    visit_type_uint32(v, name, (uint32_t *)opaque, errp);
+}
+
 static void spapr_machine_initfn(Object *obj)
 {
     sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
@@ -2686,6 +2756,11 @@ static void spapr_machine_initfn(Object *obj)
     object_property_set_description(obj, "resize-hpt",
                                     "Resizing of the Hash Page Table (enabled, disabled, required)",
                                     NULL);
+    object_property_add(obj, "vsmt", "uint32", spapr_get_vsmt,
+                        spapr_set_vsmt, NULL, &spapr->vsmt, &error_abort);
+    object_property_set_description(obj, "vsmt",
+                                    "Virtual SMT: KVM behaves as if this were"
+                                    " the host's SMT mode", &error_abort);
 }
 
 static void spapr_machine_finalizefn(Object *obj)
diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
index 86c982cf2c..68c510339f 100644
--- a/include/hw/ppc/spapr.h
+++ b/include/hw/ppc/spapr.h
@@ -99,6 +99,7 @@ struct sPAPRMachineState {
     uint64_t rtc_offset; /* Now used only during incoming migration */
     struct PPCTimebase tb;
     bool has_graphics;
+    uint32_t vsmt;       /* Virtual SMT mode (KVM's "core stride") */
 
     Notifier epow_notifier;
     QTAILQ_HEAD(, sPAPREventLogEntry) pending_events;
diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 5fc7f238eb..92a318e37b 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -74,6 +74,7 @@ static int cap_interrupt_level = false;
 static int cap_segstate;
 static int cap_booke_sregs;
 static int cap_ppc_smt;
+static int cap_ppc_smt_possible;
 static int cap_ppc_rma;
 static int cap_spapr_tce;
 static int cap_spapr_tce_64;
@@ -130,7 +131,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     cap_interrupt_level = kvm_check_extension(s, KVM_CAP_PPC_IRQ_LEVEL);
     cap_segstate = kvm_check_extension(s, KVM_CAP_PPC_SEGSTATE);
     cap_booke_sregs = kvm_check_extension(s, KVM_CAP_PPC_BOOKE_SREGS);
-    cap_ppc_smt = kvm_check_extension(s, KVM_CAP_PPC_SMT);
+    cap_ppc_smt_possible = kvm_check_extension(s, KVM_CAP_PPC_SMT_POSSIBLE);
     cap_ppc_rma = kvm_check_extension(s, KVM_CAP_PPC_RMA);
     cap_spapr_tce = kvm_check_extension(s, KVM_CAP_SPAPR_TCE);
     cap_spapr_tce_64 = kvm_check_extension(s, KVM_CAP_SPAPR_TCE_64);
@@ -144,6 +145,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
      * only activated after this by kvmppc_set_papr() */
     cap_htab_fd = kvm_check_extension(s, KVM_CAP_PPC_HTAB_FD);
     cap_fixup_hcalls = kvm_check_extension(s, KVM_CAP_PPC_FIXUP_HCALL);
+    cap_ppc_smt = kvm_vm_check_extension(s, KVM_CAP_PPC_SMT);
     cap_htm = kvm_vm_check_extension(s, KVM_CAP_PPC_HTM);
     cap_mmu_radix = kvm_vm_check_extension(s, KVM_CAP_PPC_MMU_RADIX);
     cap_mmu_hash_v3 = kvm_vm_check_extension(s, KVM_CAP_PPC_MMU_HASH_V3);
@@ -2134,6 +2136,41 @@ int kvmppc_smt_threads(void)
     return cap_ppc_smt ? cap_ppc_smt : 1;
 }
 
+int kvmppc_set_smt_threads(int smt)
+{
+    int ret;
+
+    ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_PPC_SMT, 0, smt, 0);
+    if (!ret) {
+        cap_ppc_smt = smt;
+    }
+    return ret;
+}
+
+void kvmppc_hint_smt_possible(Error **errp)
+{
+    int i;
+    GString *g;
+    char *s;
+
+    assert(kvm_enabled());
+    if (cap_ppc_smt_possible) {
+        g = g_string_new("Available VSMT modes:");
+        for (i = 63; i >= 0; i--) {
+            if ((1UL << i) & cap_ppc_smt_possible) {
+                g_string_append_printf(g, " %lu", (1UL << i));
+            }
+        }
+        s = g_string_free(g, false);
+        error_append_hint(errp, "%s.\n", s);
+        g_free(s);
+    } else {
+        error_append_hint(errp,
+                          "This KVM seems to be too old to support VSMT.\n");
+    }
+}
+
+
 #ifdef TARGET_PPC64
 off_t kvmppc_alloc_rma(void **rma)
 {
diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
index 381afe6240..e6711fc2b7 100644
--- a/target/ppc/kvm_ppc.h
+++ b/target/ppc/kvm_ppc.h
@@ -29,6 +29,8 @@ void kvmppc_set_papr(PowerPCCPU *cpu);
 int kvmppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr);
 void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy);
 int kvmppc_smt_threads(void);
+void kvmppc_hint_smt_possible(Error **errp);
+int kvmppc_set_smt_threads(int smt);
 int kvmppc_clear_tsr_bits(PowerPCCPU *cpu, uint32_t tsr_bits);
 int kvmppc_or_tsr_bits(PowerPCCPU *cpu, uint32_t tsr_bits);
 int kvmppc_set_tcr(PowerPCCPU *cpu);
@@ -148,6 +150,16 @@ static inline int kvmppc_smt_threads(void)
     return 1;
 }
 
+static inline void kvmppc_hint_smt_possible(Error **errp)
+{
+    return;
+}
+
+static inline int kvmppc_set_smt_threads(int smt)
+{
+    return 0;
+}
+
 static inline int kvmppc_or_tsr_bits(PowerPCCPU *cpu, uint32_t tsr_bits)
 {
     return 0;
diff --git a/target/ppc/translate_init.c b/target/ppc/translate_init.c
index 0ce923be46..7f6a349e43 100644
--- a/target/ppc/translate_init.c
+++ b/target/ppc/translate_init.c
@@ -9907,20 +9907,6 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
     int max_smt = kvmppc_smt_threads();
 #endif
 
-#if !defined(CONFIG_USER_ONLY)
-    if (smp_threads > max_smt) {
-        error_setg(errp, "Cannot support more than %d threads on PPC with %s",
-                   max_smt, kvm_enabled() ? "KVM" : "TCG");
-        return;
-    }
-    if (!is_power_of_2(smp_threads)) {
-        error_setg(errp, "Cannot support %d threads on PPC with %s, "
-                   "threads count must be a power of 2.",
-                   smp_threads, kvm_enabled() ? "KVM" : "TCG");
-        return;
-    }
-#endif
-
     cpu_exec_realizefn(cs, &local_err);
     if (local_err != NULL) {
         error_propagate(errp, local_err);
-- 
2.14.1.2.g4274c698f

Re: [Qemu-devel] [PATCH v4 1/1] PPC: KVM: Support machine option to set VSMT mode

[David Gibson]

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On Fri, Aug 18, 2017 at 03:50:22PM +1000, Sam Bobroff wrote:
> KVM now allows writing to KVM_CAP_PPC_SMT which has previously been
> read only. Doing so causes KVM to act, for that VM, as if the host's
> SMT mode was the given value. This is particularly important on Power
> 9 systems because their default value is 1, but they are able to
> support values up to 8.
> 
> This patch introduces a way to control this capability via a new
> machine property called VSMT ("Virtual SMT"). If the value is not set
> on the command line a default is chosen that is, when possible,
> compatible with legacy systems.
> 
> Note that the intialization of KVM_CAP_PPC_SMT has changed slightly
> because it has changed (in KVM) from a global capability to a
> VM-specific one. This won't cause a problem on older KVMs because VM
> capabilities fall back to global ones.
> 
> Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>

There are some things that could be tweaked, but I see no reason they
can't be done as followups.

Applied to ppc-for-2.11.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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