[PATCH v2 0/3] nvdimm: read-only file support

[PATCH v2 0/3] nvdimm: read-only file support

[Stefan Hajnoczi]

v2:
 * s/the the/the/ in documentation [Philippe]
 * Assign nvdimm->unarmed earlier [Philippe]

There is currently no way to back an NVDIMM with a read-only file so it can be
safely shared between untrusted guests.

Introduce an -object memory-backend-file,readonly=on|off option.

Julio Montes sent an earlier patch here:
https://patchew.org/QEMU/20190708211936.8037-1-julio.montes@intel.com/

Eric Ernst requested this feature again for Kata Containers so I gave it a try.

Stefan Hajnoczi (3):
  memory: add readonly support to memory_region_init_ram_from_file()
  hostmem-file: add readonly=on|off option
  nvdimm: honor -object memory-backend-file,readonly=on option

 docs/nvdimm.txt           |  8 +++++++-
 include/exec/memory.h     |  2 ++
 include/exec/ram_addr.h   |  5 +++--
 include/qemu/mmap-alloc.h |  2 ++
 backends/hostmem-file.c   | 26 +++++++++++++++++++++++++-
 exec.c                    | 18 +++++++++++-------
 hw/mem/nvdimm.c           |  4 ++++
 softmmu/memory.c          |  7 +++++--
 util/mmap-alloc.c         | 10 ++++++----
 util/oslib-posix.c        |  2 +-
 qemu-options.hx           |  5 ++++-
 11 files changed, 70 insertions(+), 19 deletions(-)

-- 
2.26.2

[PATCH v2 1/3] memory: add readonly support to memory_region_init_ram_from_file()

[Stefan Hajnoczi]

There is currently no way to open(O_RDONLY) and mmap(PROT_READ) when
creating a memory region from a file. This functionality is needed since
the underlying host file may not allow writing.

Add a bool readonly argument to memory_region_init_ram_from_file() and
the APIs it calls.

Extend memory_region_init_ram_from_file() rather than introducing a
memory_region_init_rom_from_file() API so that callers can easily make a
choice between read/write and read-only at runtime without calling
different APIs.

No new RAMBlock flag is introduced for read-only because it's unclear
whether RAMBlocks need to know that they are read-only. Pass a bool
readonly argument instead.

Both of these design decisions can be changed in the future. It just
seemed like the simplest approach to me.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/exec/memory.h     |  2 ++
 include/exec/ram_addr.h   |  5 +++--
 include/qemu/mmap-alloc.h |  2 ++
 backends/hostmem-file.c   |  2 +-
 exec.c                    | 18 +++++++++++-------
 softmmu/memory.c          |  7 +++++--
 util/mmap-alloc.c         | 10 ++++++----
 util/oslib-posix.c        |  2 +-
 8 files changed, 31 insertions(+), 17 deletions(-)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index f1bb2a7df5..a81fa26165 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -879,6 +879,7 @@ void memory_region_init_resizeable_ram(MemoryRegion *mr,
  *             - RAM_PMEM: the memory is persistent memory
  *             Other bits are ignored now.
  * @path: the path in which to allocate the RAM.
+ * @readonly: true to open @path for reading, false for read/write.
  * @errp: pointer to Error*, to store an error if it happens.
  *
  * Note that this function does not do anything to cause the data in the
@@ -891,6 +892,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       uint64_t align,
                                       uint32_t ram_flags,
                                       const char *path,
+                                      bool readonly,
                                       Error **errp);
 
 /**
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 3ef729a23c..2a0360a0f2 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -110,6 +110,7 @@ long qemu_maxrampagesize(void);
  *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
  *              Other bits are ignored.
  *  @mem_path or @fd: specify the backing file or device
+ *  @readonly: true to open @path for reading, false for read/write.
  *  @errp: pointer to Error*, to store an error if it happens
  *
  * Return:
@@ -118,9 +119,9 @@ long qemu_maxrampagesize(void);
  */
 RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
                                    uint32_t ram_flags, const char *mem_path,
-                                   Error **errp);
+                                   bool readonly, Error **errp);
 RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
-                                 uint32_t ram_flags, int fd,
+                                 uint32_t ram_flags, int fd, bool readonly,
                                  Error **errp);
 
 RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
index e786266b92..8b7a5c70f3 100644
--- a/include/qemu/mmap-alloc.h
+++ b/include/qemu/mmap-alloc.h
@@ -14,6 +14,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
  *  @size: the number of bytes to be mmaped
  *  @align: if not zero, specify the alignment of the starting mapping address;
  *          otherwise, the alignment in use will be determined by QEMU.
+ *  @readonly: true for a read-only mapping, false for read/write.
  *  @shared: map has RAM_SHARED flag.
  *  @is_pmem: map has RAM_PMEM flag.
  *
@@ -24,6 +25,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
+                    bool readonly,
                     bool shared,
                     bool is_pmem);
 
diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index a3b2e8209e..dffdf142e0 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -58,7 +58,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                      backend->size, fb->align,
                                      (backend->share ? RAM_SHARED : 0) |
                                      (fb->is_pmem ? RAM_PMEM : 0),
-                                     fb->mem_path, errp);
+                                     fb->mem_path, false, errp);
     g_free(name);
 #endif
 }
diff --git a/exec.c b/exec.c
index e34b602bdf..f1e82dad7a 100644
--- a/exec.c
+++ b/exec.c
@@ -1770,6 +1770,7 @@ static int64_t get_file_align(int fd)
 
 static int file_ram_open(const char *path,
                          const char *region_name,
+                         bool readonly,
                          bool *created,
                          Error **errp)
 {
@@ -1780,7 +1781,7 @@ static int file_ram_open(const char *path,
 
     *created = false;
     for (;;) {
-        fd = open(path, O_RDWR);
+        fd = open(path, readonly ? O_RDONLY : O_RDWR);
         if (fd >= 0) {
             /* @path names an existing file, use it */
             break;
@@ -1832,6 +1833,7 @@ static int file_ram_open(const char *path,
 static void *file_ram_alloc(RAMBlock *block,
                             ram_addr_t memory,
                             int fd,
+                            bool readonly,
                             bool truncate,
                             Error **errp)
 {
@@ -1882,7 +1884,7 @@ static void *file_ram_alloc(RAMBlock *block,
         perror("ftruncate");
     }
 
-    area = qemu_ram_mmap(fd, memory, block->mr->align,
+    area = qemu_ram_mmap(fd, memory, block->mr->align, readonly,
                          block->flags & RAM_SHARED, block->flags & RAM_PMEM);
     if (area == MAP_FAILED) {
         error_setg_errno(errp, errno,
@@ -2314,7 +2316,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp, bool shared)
 
 #ifdef CONFIG_POSIX
 RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
-                                 uint32_t ram_flags, int fd,
+                                 uint32_t ram_flags, int fd, bool readonly,
                                  Error **errp)
 {
     RAMBlock *new_block;
@@ -2368,7 +2370,8 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
     new_block->used_length = size;
     new_block->max_length = size;
     new_block->flags = ram_flags;
-    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, errp);
+    new_block->host = file_ram_alloc(new_block, size, fd, readonly,
+                                     !file_size, errp);
     if (!new_block->host) {
         g_free(new_block);
         return NULL;
@@ -2387,18 +2390,19 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
 
 RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
                                    uint32_t ram_flags, const char *mem_path,
-                                   Error **errp)
+                                   bool readonly, Error **errp)
 {
     int fd;
     bool created;
     RAMBlock *block;
 
-    fd = file_ram_open(mem_path, memory_region_name(mr), &created, errp);
+    fd = file_ram_open(mem_path, memory_region_name(mr), readonly, &created,
+                       errp);
     if (fd < 0) {
         return NULL;
     }
 
-    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, errp);
+    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, readonly, errp);
     if (!block) {
         if (created) {
             unlink(mem_path);
diff --git a/softmmu/memory.c b/softmmu/memory.c
index d030eb6f7c..1b0d1d42c6 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -1553,15 +1553,18 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       uint64_t align,
                                       uint32_t ram_flags,
                                       const char *path,
+                                      bool readonly,
                                       Error **errp)
 {
     Error *err = NULL;
     memory_region_init(mr, owner, name, size);
     mr->ram = true;
+    mr->readonly = readonly;
     mr->terminates = true;
     mr->destructor = memory_region_destructor_ram;
     mr->align = align;
-    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path, &err);
+    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
+                                             readonly, &err);
     mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
     if (err) {
         mr->size = int128_zero();
@@ -1585,7 +1588,7 @@ void memory_region_init_ram_from_fd(MemoryRegion *mr,
     mr->destructor = memory_region_destructor_ram;
     mr->ram_block = qemu_ram_alloc_from_fd(size, mr,
                                            share ? RAM_SHARED : 0,
-                                           fd, &err);
+                                           fd, false, &err);
     mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
     if (err) {
         mr->size = int128_zero();
diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 27dcccd8ec..890fda6a35 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -85,9 +85,11 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
+                    bool readonly,
                     bool shared,
                     bool is_pmem)
 {
+    int prot;
     int flags;
     int map_sync_flags = 0;
     int guardfd;
@@ -146,8 +148,9 @@ void *qemu_ram_mmap(int fd,
 
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
-    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
-               flags | map_sync_flags, fd, 0);
+    prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
+
+    ptr = mmap(guardptr + offset, size, prot, flags | map_sync_flags, fd, 0);
 
     if (ptr == MAP_FAILED && map_sync_flags) {
         if (errno == ENOTSUP) {
@@ -171,8 +174,7 @@ void *qemu_ram_mmap(int fd,
          * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
          * we will remove these flags to handle compatibility.
          */
-        ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
-                   flags, fd, 0);
+        ptr = mmap(guardptr + offset, size, prot, flags, fd, 0);
     }
 
     if (ptr == MAP_FAILED) {
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index ad8001a4ad..236b3a88c1 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -227,7 +227,7 @@ void *qemu_memalign(size_t alignment, size_t size)
 void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared)
 {
     size_t align = QEMU_VMALLOC_ALIGN;
-    void *ptr = qemu_ram_mmap(-1, size, align, shared, false);
+    void *ptr = qemu_ram_mmap(-1, size, align, false, shared, false);
 
     if (ptr == MAP_FAILED) {
         return NULL;
-- 
2.26.2

[PATCH v2 2/3] hostmem-file: add readonly=on|off option

[Stefan Hajnoczi]

Let -object memory-backend-file work on read-only files when the
readonly=on option is given. This can be used to share the contents of a
file between multiple guests while preventing them from consuming
Copy-on-Write memory if guests dirty the pages, for example.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 backends/hostmem-file.c | 26 +++++++++++++++++++++++++-
 qemu-options.hx         |  5 ++++-
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index dffdf142e0..da585e4300 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -31,6 +31,7 @@ struct HostMemoryBackendFile {
     uint64_t align;
     bool discard_data;
     bool is_pmem;
+    bool readonly;
 };
 
 static void
@@ -58,7 +59,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                      backend->size, fb->align,
                                      (backend->share ? RAM_SHARED : 0) |
                                      (fb->is_pmem ? RAM_PMEM : 0),
-                                     fb->mem_path, false, errp);
+                                     fb->mem_path, fb->readonly, errp);
     g_free(name);
 #endif
 }
@@ -153,6 +154,26 @@ static void file_memory_backend_set_pmem(Object *o, bool value, Error **errp)
     fb->is_pmem = value;
 }
 
+static bool file_memory_backend_get_readonly(Object *o, Error **errp)
+{
+    return MEMORY_BACKEND_FILE(o)->readonly;
+}
+
+static void file_memory_backend_set_readonly(Object *o, bool value,
+                                             Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property 'readonly' of %s.",
+                   object_get_typename(o));
+        return;
+    }
+
+    fb->readonly = value;
+}
+
 static void file_backend_unparent(Object *obj)
 {
     HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -184,6 +205,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
         NULL, NULL);
     object_class_property_add_bool(oc, "pmem",
         file_memory_backend_get_pmem, file_memory_backend_set_pmem);
+    object_class_property_add_bool(oc, "readonly",
+        file_memory_backend_get_readonly,
+        file_memory_backend_set_readonly);
 }
 
 static void file_backend_instance_finalize(Object *o)
diff --git a/qemu-options.hx b/qemu-options.hx
index b0f020594e..3dfaaddd62 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4369,7 +4369,7 @@ SRST
     they are specified. Note that the 'id' property must be set. These
     objects are placed in the '/objects' path.
 
-    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align``
+    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,readonly=on|off``
         Creates a memory file backend object, which can be used to back
         the guest RAM with huge pages.
 
@@ -4452,6 +4452,9 @@ SRST
         4.15) and the filesystem of ``mem-path`` mounted with DAX
         option.
 
+        The ``readonly`` option specifies whether the backing file is opened
+        read-only or read-write (default).
+
     ``-object memory-backend-ram,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
         Creates a memory backend object, which can be used to back the
         guest RAM. Memory backend objects offer more control than the
-- 
2.26.2

[PATCH v2 3/3] nvdimm: honor -object memory-backend-file, readonly=on option

[Stefan Hajnoczi]

Make it possible to present read-only files to the guest as "unarmed"
NVDIMMs. The Linux NVDIMM device (/dev/pmemX) is read-only.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 docs/nvdimm.txt | 8 +++++++-
 hw/mem/nvdimm.c | 4 ++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index c2c6e441b3..06c2008107 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -17,7 +17,7 @@ following command line options:
 
  -machine pc,nvdimm
  -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
- -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE
+ -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE,readonly=off
  -device nvdimm,id=nvdimm1,memdev=mem1
 
 Where,
@@ -42,6 +42,12 @@ Where,
    "share=off", then guest writes won't be applied to the backend
    file and thus will be invisible to other guests.
 
+   "readonly=on/off" controls whether the file $PATH is opened read-only or
+   read/write (default). "readonly=on" sets the ACPI NFIT NVDIMM Region Mapping
+   Structure "NVDIMM State Flags" Bit 3 indicating that the device is "unarmed"
+   and cannot accept persistent writes. Linux guest drivers set the device to
+   read-only when this bit is present.
+
  - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM
    device whose storage is provided by above memory backend device.
 
diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
index e1574bc07c..848cd65917 100644
--- a/hw/mem/nvdimm.c
+++ b/hw/mem/nvdimm.c
@@ -146,6 +146,10 @@ static void nvdimm_prepare_memory_region(NVDIMMDevice *nvdimm, Error **errp)
         return;
     }
 
+    if (memory_region_is_rom(mr)) {
+        nvdimm->unarmed = true; /* this device is read-only */
+    }
+
     nvdimm->nvdimm_mr = g_new(MemoryRegion, 1);
     memory_region_init_alias(nvdimm->nvdimm_mr, OBJECT(dimm),
                              "nvdimm-memory", mr, 0, pmem_size);
-- 
2.26.2

Re: [PATCH v2 0/3] nvdimm: read-only file support

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1368 bytes --]

On Wed, Sep 16, 2020 at 10:51:47AM +0100, Stefan Hajnoczi wrote:
> v2:
>  * s/the the/the/ in documentation [Philippe]
>  * Assign nvdimm->unarmed earlier [Philippe]

Ping

> There is currently no way to back an NVDIMM with a read-only file so it can be
> safely shared between untrusted guests.
> 
> Introduce an -object memory-backend-file,readonly=on|off option.
> 
> Julio Montes sent an earlier patch here:
> https://patchew.org/QEMU/20190708211936.8037-1-julio.montes@intel.com/
> 
> Eric Ernst requested this feature again for Kata Containers so I gave it a try.
> 
> Stefan Hajnoczi (3):
>   memory: add readonly support to memory_region_init_ram_from_file()
>   hostmem-file: add readonly=on|off option
>   nvdimm: honor -object memory-backend-file,readonly=on option
> 
>  docs/nvdimm.txt           |  8 +++++++-
>  include/exec/memory.h     |  2 ++
>  include/exec/ram_addr.h   |  5 +++--
>  include/qemu/mmap-alloc.h |  2 ++
>  backends/hostmem-file.c   | 26 +++++++++++++++++++++++++-
>  exec.c                    | 18 +++++++++++-------
>  hw/mem/nvdimm.c           |  4 ++++
>  softmmu/memory.c          |  7 +++++--
>  util/mmap-alloc.c         | 10 ++++++----
>  util/oslib-posix.c        |  2 +-
>  qemu-options.hx           |  5 ++++-
>  11 files changed, 70 insertions(+), 19 deletions(-)
> 
> -- 
> 2.26.2
> 

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 484 bytes --]

Re: [PATCH v2 0/3] nvdimm: read-only file support

[Liam Merwick]

On 16/09/2020 10:51, Stefan Hajnoczi wrote:
> v2:
>   * s/the the/the/ in documentation [Philippe]
>   * Assign nvdimm->unarmed earlier [Philippe]
> 
> There is currently no way to back an NVDIMM with a read-only file so it can be
> safely shared between untrusted guests.
> 
> Introduce an -object memory-backend-file,readonly=on|off option.
> 
> Julio Montes sent an earlier patch here:
> https://patchew.org/QEMU/20190708211936.8037-1-julio.montes@intel.com/
> 
> Eric Ernst requested this feature again for Kata Containers so I gave it a try.
> 
> Stefan Hajnoczi (3):
>    memory: add readonly support to memory_region_init_ram_from_file()
>    hostmem-file: add readonly=on|off option
>    nvdimm: honor -object memory-backend-file,readonly=on option
> 


Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>

(I just quickly modified kata-runtime to unconditionally pass 
readonly=on and verified that the root filesystem couldn't be remounted 
as rw)

Is this a candidate for 6.0?

Regards,
Liam


>   docs/nvdimm.txt           |  8 +++++++-
>   include/exec/memory.h     |  2 ++
>   include/exec/ram_addr.h   |  5 +++--
>   include/qemu/mmap-alloc.h |  2 ++
>   backends/hostmem-file.c   | 26 +++++++++++++++++++++++++-
>   exec.c                    | 18 +++++++++++-------
>   hw/mem/nvdimm.c           |  4 ++++
>   softmmu/memory.c          |  7 +++++--
>   util/mmap-alloc.c         | 10 ++++++----
>   util/oslib-posix.c        |  2 +-
>   qemu-options.hx           |  5 ++++-
>   11 files changed, 70 insertions(+), 19 deletions(-)
> 

Re: [PATCH v2 1/3] memory: add readonly support to memory_region_init_ram_from_file()

[Igor Mammedov]

On Wed, 16 Sep 2020 10:51:48 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> There is currently no way to open(O_RDONLY) and mmap(PROT_READ) when
> creating a memory region from a file. This functionality is needed since
> the underlying host file may not allow writing.
> 
> Add a bool readonly argument to memory_region_init_ram_from_file() and
> the APIs it calls.
> 
> Extend memory_region_init_ram_from_file() rather than introducing a
> memory_region_init_rom_from_file() API so that callers can easily make a
> choice between read/write and read-only at runtime without calling
> different APIs.
> 
> No new RAMBlock flag is introduced for read-only because it's unclear
> whether RAMBlocks need to know that they are read-only. Pass a bool
> readonly argument instead.
> 
> Both of these design decisions can be changed in the future. It just
> seemed like the simplest approach to me.
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  include/exec/memory.h     |  2 ++
>  include/exec/ram_addr.h   |  5 +++--
>  include/qemu/mmap-alloc.h |  2 ++
>  backends/hostmem-file.c   |  2 +-
>  exec.c                    | 18 +++++++++++-------
>  softmmu/memory.c          |  7 +++++--
>  util/mmap-alloc.c         | 10 ++++++----
>  util/oslib-posix.c        |  2 +-
>  8 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index f1bb2a7df5..a81fa26165 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -879,6 +879,7 @@ void memory_region_init_resizeable_ram(MemoryRegion *mr,
>   *             - RAM_PMEM: the memory is persistent memory
>   *             Other bits are ignored now.
>   * @path: the path in which to allocate the RAM.
> + * @readonly: true to open @path for reading, false for read/write.
>   * @errp: pointer to Error*, to store an error if it happens.
>   *
>   * Note that this function does not do anything to cause the data in the
> @@ -891,6 +892,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>                                        uint64_t align,
>                                        uint32_t ram_flags,
>                                        const char *path,
> +                                      bool readonly,
>                                        Error **errp);
>  
>  /**
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 3ef729a23c..2a0360a0f2 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -110,6 +110,7 @@ long qemu_maxrampagesize(void);
>   *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
>   *              Other bits are ignored.
>   *  @mem_path or @fd: specify the backing file or device
> + *  @readonly: true to open @path for reading, false for read/write.
>   *  @errp: pointer to Error*, to store an error if it happens
>   *
>   * Return:
> @@ -118,9 +119,9 @@ long qemu_maxrampagesize(void);
>   */
>  RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>                                     uint32_t ram_flags, const char *mem_path,
> -                                   Error **errp);
> +                                   bool readonly, Error **errp);
>  RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
> -                                 uint32_t ram_flags, int fd,
> +                                 uint32_t ram_flags, int fd, bool readonly,
>                                   Error **errp);
>  
>  RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
> diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
> index e786266b92..8b7a5c70f3 100644
> --- a/include/qemu/mmap-alloc.h
> +++ b/include/qemu/mmap-alloc.h
> @@ -14,6 +14,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
>   *  @size: the number of bytes to be mmaped
>   *  @align: if not zero, specify the alignment of the starting mapping address;
>   *          otherwise, the alignment in use will be determined by QEMU.
> + *  @readonly: true for a read-only mapping, false for read/write.
>   *  @shared: map has RAM_SHARED flag.
>   *  @is_pmem: map has RAM_PMEM flag.
>   *
> @@ -24,6 +25,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
>  void *qemu_ram_mmap(int fd,
>                      size_t size,
>                      size_t align,
> +                    bool readonly,
>                      bool shared,
>                      bool is_pmem);
>  
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index a3b2e8209e..dffdf142e0 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -58,7 +58,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>                                       backend->size, fb->align,
>                                       (backend->share ? RAM_SHARED : 0) |
>                                       (fb->is_pmem ? RAM_PMEM : 0),
> -                                     fb->mem_path, errp);
> +                                     fb->mem_path, false, errp);
>      g_free(name);
>  #endif
>  }
> diff --git a/exec.c b/exec.c
> index e34b602bdf..f1e82dad7a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1770,6 +1770,7 @@ static int64_t get_file_align(int fd)
>  
>  static int file_ram_open(const char *path,
>                           const char *region_name,
> +                         bool readonly,
>                           bool *created,
>                           Error **errp)
>  {
> @@ -1780,7 +1781,7 @@ static int file_ram_open(const char *path,
>  
>      *created = false;
>      for (;;) {
> -        fd = open(path, O_RDWR);
> +        fd = open(path, readonly ? O_RDONLY : O_RDWR);
>          if (fd >= 0) {
>              /* @path names an existing file, use it */
>              break;
> @@ -1832,6 +1833,7 @@ static int file_ram_open(const char *path,
>  static void *file_ram_alloc(RAMBlock *block,
>                              ram_addr_t memory,
>                              int fd,
> +                            bool readonly,
>                              bool truncate,
>                              Error **errp)
>  {
> @@ -1882,7 +1884,7 @@ static void *file_ram_alloc(RAMBlock *block,
>          perror("ftruncate");
>      }
>  
> -    area = qemu_ram_mmap(fd, memory, block->mr->align,
> +    area = qemu_ram_mmap(fd, memory, block->mr->align, readonly,
>                           block->flags & RAM_SHARED, block->flags & RAM_PMEM);
>      if (area == MAP_FAILED) {
>          error_setg_errno(errp, errno,
> @@ -2314,7 +2316,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp, bool shared)
>  
>  #ifdef CONFIG_POSIX
>  RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
> -                                 uint32_t ram_flags, int fd,
> +                                 uint32_t ram_flags, int fd, bool readonly,
>                                   Error **errp)
>  {
>      RAMBlock *new_block;
> @@ -2368,7 +2370,8 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>      new_block->used_length = size;
>      new_block->max_length = size;
>      new_block->flags = ram_flags;
> -    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, errp);
> +    new_block->host = file_ram_alloc(new_block, size, fd, readonly,
> +                                     !file_size, errp);
>      if (!new_block->host) {
>          g_free(new_block);
>          return NULL;
> @@ -2387,18 +2390,19 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>  
>  RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>                                     uint32_t ram_flags, const char *mem_path,
> -                                   Error **errp)
> +                                   bool readonly, Error **errp)
>  {
>      int fd;
>      bool created;
>      RAMBlock *block;
>  
> -    fd = file_ram_open(mem_path, memory_region_name(mr), &created, errp);
> +    fd = file_ram_open(mem_path, memory_region_name(mr), readonly, &created,
> +                       errp);
>      if (fd < 0) {
>          return NULL;
>      }
>  
> -    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, errp);
> +    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, readonly, errp);
>      if (!block) {
>          if (created) {
>              unlink(mem_path);
> diff --git a/softmmu/memory.c b/softmmu/memory.c
> index d030eb6f7c..1b0d1d42c6 100644
> --- a/softmmu/memory.c
> +++ b/softmmu/memory.c
> @@ -1553,15 +1553,18 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>                                        uint64_t align,
>                                        uint32_t ram_flags,
>                                        const char *path,
> +                                      bool readonly,
>                                        Error **errp)
>  {
>      Error *err = NULL;
>      memory_region_init(mr, owner, name, size);
>      mr->ram = true;
> +    mr->readonly = readonly;
>      mr->terminates = true;
>      mr->destructor = memory_region_destructor_ram;
>      mr->align = align;
> -    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path, &err);
> +    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
> +                                             readonly, &err);
>      mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>      if (err) {
>          mr->size = int128_zero();
> @@ -1585,7 +1588,7 @@ void memory_region_init_ram_from_fd(MemoryRegion *mr,
>      mr->destructor = memory_region_destructor_ram;
>      mr->ram_block = qemu_ram_alloc_from_fd(size, mr,
>                                             share ? RAM_SHARED : 0,
> -                                           fd, &err);
> +                                           fd, false, &err);
>      mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>      if (err) {
>          mr->size = int128_zero();
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 27dcccd8ec..890fda6a35 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -85,9 +85,11 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>  void *qemu_ram_mmap(int fd,
>                      size_t size,
>                      size_t align,
> +                    bool readonly,
>                      bool shared,
>                      bool is_pmem)
>  {
> +    int prot;
>      int flags;
>      int map_sync_flags = 0;
>      int guardfd;
> @@ -146,8 +148,9 @@ void *qemu_ram_mmap(int fd,
>  
>      offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
>  
> -    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
> -               flags | map_sync_flags, fd, 0);
> +    prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
> +
> +    ptr = mmap(guardptr + offset, size, prot, flags | map_sync_flags, fd, 0);
>  
>      if (ptr == MAP_FAILED && map_sync_flags) {
>          if (errno == ENOTSUP) {
> @@ -171,8 +174,7 @@ void *qemu_ram_mmap(int fd,
>           * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
>           * we will remove these flags to handle compatibility.
>           */
> -        ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
> -                   flags, fd, 0);
> +        ptr = mmap(guardptr + offset, size, prot, flags, fd, 0);
>      }
>  
>      if (ptr == MAP_FAILED) {
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index ad8001a4ad..236b3a88c1 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -227,7 +227,7 @@ void *qemu_memalign(size_t alignment, size_t size)
>  void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared)
>  {
>      size_t align = QEMU_VMALLOC_ALIGN;
> -    void *ptr = qemu_ram_mmap(-1, size, align, shared, false);
> +    void *ptr = qemu_ram_mmap(-1, size, align, false, shared, false);
>  
>      if (ptr == MAP_FAILED) {
>          return NULL;

Re: [PATCH v2 2/3] hostmem-file: add readonly=on|off option

[Igor Mammedov]

On Wed, 16 Sep 2020 10:51:49 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> Let -object memory-backend-file work on read-only files when the
> readonly=on option is given. This can be used to share the contents of a
> file between multiple guests while preventing them from consuming
> Copy-on-Write memory if guests dirty the pages, for example.
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

cosmetic/style nits only

s/Object *o/Object *obj/

for consistency with the rest of the code in file.

> ---
>  backends/hostmem-file.c | 26 +++++++++++++++++++++++++-
>  qemu-options.hx         |  5 ++++-
>  2 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index dffdf142e0..da585e4300 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -31,6 +31,7 @@ struct HostMemoryBackendFile {
>      uint64_t align;
>      bool discard_data;
>      bool is_pmem;
> +    bool readonly;
>  };
>  
>  static void
> @@ -58,7 +59,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>                                       backend->size, fb->align,
>                                       (backend->share ? RAM_SHARED : 0) |
>                                       (fb->is_pmem ? RAM_PMEM : 0),
> -                                     fb->mem_path, false, errp);
> +                                     fb->mem_path, fb->readonly, errp);
>      g_free(name);
>  #endif
>  }
> @@ -153,6 +154,26 @@ static void file_memory_backend_set_pmem(Object *o, bool value, Error **errp)
>      fb->is_pmem = value;
>  }
>  
> +static bool file_memory_backend_get_readonly(Object *o, Error **errp)
> +{
> +    return MEMORY_BACKEND_FILE(o)->readonly;

I thought using macro this way not acceptable and one should use

HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);

return fb->readonly;


> +}
> +
> +static void file_memory_backend_set_readonly(Object *o, bool value,
> +                                             Error **errp)
> +{
> +    HostMemoryBackend *backend = MEMORY_BACKEND(o);
> +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
> +
> +    if (host_memory_backend_mr_inited(backend)) {
> +        error_setg(errp, "cannot change property 'readonly' of %s.",
> +                   object_get_typename(o));
> +        return;
> +    }
> +
> +    fb->readonly = value;
> +}
> +
>  static void file_backend_unparent(Object *obj)
>  {
>      HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> @@ -184,6 +205,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
>          NULL, NULL);
>      object_class_property_add_bool(oc, "pmem",
>          file_memory_backend_get_pmem, file_memory_backend_set_pmem);
> +    object_class_property_add_bool(oc, "readonly",
> +        file_memory_backend_get_readonly,
> +        file_memory_backend_set_readonly);
>  }
>  
>  static void file_backend_instance_finalize(Object *o)
> diff --git a/qemu-options.hx b/qemu-options.hx
> index b0f020594e..3dfaaddd62 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4369,7 +4369,7 @@ SRST
>      they are specified. Note that the 'id' property must be set. These
>      objects are placed in the '/objects' path.
>  
> -    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align``
> +    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,readonly=on|off``
>          Creates a memory file backend object, which can be used to back
>          the guest RAM with huge pages.
>  
> @@ -4452,6 +4452,9 @@ SRST
>          4.15) and the filesystem of ``mem-path`` mounted with DAX
>          option.
>  
> +        The ``readonly`` option specifies whether the backing file is opened
> +        read-only or read-write (default).
> +
>      ``-object memory-backend-ram,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
>          Creates a memory backend object, which can be used to back the
>          guest RAM. Memory backend objects offer more control than the

Re: [PATCH v2 3/3] nvdimm: honor -object memory-backend-file, readonly=on option

[Igor Mammedov]

On Wed, 16 Sep 2020 10:51:50 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> Make it possible to present read-only files to the guest as "unarmed"
> NVDIMMs. The Linux NVDIMM device (/dev/pmemX) is read-only.
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/nvdimm.txt | 8 +++++++-
>  hw/mem/nvdimm.c | 4 ++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index c2c6e441b3..06c2008107 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -17,7 +17,7 @@ following command line options:
>  
>   -machine pc,nvdimm
>   -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
> - -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE
> + -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE,readonly=off
>   -device nvdimm,id=nvdimm1,memdev=mem1
>  
>  Where,
> @@ -42,6 +42,12 @@ Where,
>     "share=off", then guest writes won't be applied to the backend
>     file and thus will be invisible to other guests.
>  
> +   "readonly=on/off" controls whether the file $PATH is opened read-only or
> +   read/write (default). "readonly=on" sets the ACPI NFIT NVDIMM Region Mapping
> +   Structure "NVDIMM State Flags" Bit 3 indicating that the device is "unarmed"
> +   and cannot accept persistent writes. Linux guest drivers set the device to
> +   read-only when this bit is present.
> +
>   - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM
>     device whose storage is provided by above memory backend device.
>  
> diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
> index e1574bc07c..848cd65917 100644
> --- a/hw/mem/nvdimm.c
> +++ b/hw/mem/nvdimm.c
> @@ -146,6 +146,10 @@ static void nvdimm_prepare_memory_region(NVDIMMDevice *nvdimm, Error **errp)
>          return;
>      }
>  
> +    if (memory_region_is_rom(mr)) {
> +        nvdimm->unarmed = true; /* this device is read-only */
> +    }
we probably should error out in case 'backend,readonly=true + nvdimm,unarmed=false'
instead of silently ignoring user's input

>      nvdimm->nvdimm_mr = g_new(MemoryRegion, 1);
>      memory_region_init_alias(nvdimm->nvdimm_mr, OBJECT(dimm),
>                               "nvdimm-memory", mr, 0, pmem_size);

Re: [PATCH v2 2/3] hostmem-file: add readonly=on|off option

[Stefan Hajnoczi]

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On Mon, Dec 14, 2020 at 12:10:15PM +0100, Igor Mammedov wrote:
> On Wed, 16 Sep 2020 10:51:49 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > Let -object memory-backend-file work on read-only files when the
> > readonly=on option is given. This can be used to share the contents of a
> > file between multiple guests while preventing them from consuming
> > Copy-on-Write memory if guests dirty the pages, for example.
> > 
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> cosmetic/style nits only
> 
> s/Object *o/Object *obj/
> 
> for consistency with the rest of the code in file.

Will fix.

> > @@ -153,6 +154,26 @@ static void file_memory_backend_set_pmem(Object *o, bool value, Error **errp)
> >      fb->is_pmem = value;
> >  }
> >  
> > +static bool file_memory_backend_get_readonly(Object *o, Error **errp)
> > +{
> > +    return MEMORY_BACKEND_FILE(o)->readonly;
> 
> I thought using macro this way not acceptable and one should use
> 
> HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
> 
> return fb->readonly;

I'm not sure where this is forbidden or why? I've updated the patch as
suggested anyway.

Stefan

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Re: [PATCH v2 0/3] nvdimm: read-only file support

[Stefan Hajnoczi]

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On Thu, Dec 10, 2020 at 04:48:57PM +0000, Liam Merwick wrote:
> On 16/09/2020 10:51, Stefan Hajnoczi wrote:
> > v2:
> >   * s/the the/the/ in documentation [Philippe]
> >   * Assign nvdimm->unarmed earlier [Philippe]
> > 
> > There is currently no way to back an NVDIMM with a read-only file so it can be
> > safely shared between untrusted guests.
> > 
> > Introduce an -object memory-backend-file,readonly=on|off option.
> > 
> > Julio Montes sent an earlier patch here:
> > https://patchew.org/QEMU/20190708211936.8037-1-julio.montes@intel.com/
> > 
> > Eric Ernst requested this feature again for Kata Containers so I gave it a try.
> > 
> > Stefan Hajnoczi (3):
> >    memory: add readonly support to memory_region_init_ram_from_file()
> >    hostmem-file: add readonly=on|off option
> >    nvdimm: honor -object memory-backend-file,readonly=on option
> > 
> 
> 
> Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
> Tested-by: Liam Merwick <liam.merwick@oracle.com>
> 
> (I just quickly modified kata-runtime to unconditionally pass readonly=on
> and verified that the root filesystem couldn't be remounted as rw)
> 
> Is this a candidate for 6.0?

Thanks! Yes, this is for 6.0.

Stefan

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Re: [PATCH v2 3/3] nvdimm: honor -object memory-backend-file, readonly=on option

[Stefan Hajnoczi]

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On Mon, Dec 14, 2020 at 12:19:05PM +0100, Igor Mammedov wrote:
> On Wed, 16 Sep 2020 10:51:50 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
> > index e1574bc07c..848cd65917 100644
> > --- a/hw/mem/nvdimm.c
> > +++ b/hw/mem/nvdimm.c
> > @@ -146,6 +146,10 @@ static void nvdimm_prepare_memory_region(NVDIMMDevice *nvdimm, Error **errp)
> >          return;
> >      }
> >  
> > +    if (memory_region_is_rom(mr)) {
> > +        nvdimm->unarmed = true; /* this device is read-only */
> > +    }
> we probably should error out in case 'backend,readonly=true + nvdimm,unarmed=false'
> instead of silently ignoring user's input

Will fix.

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Re: [PATCH v2 2/3] hostmem-file: add readonly=on|off option

[Eduardo Habkost]

On Mon, Jan 04, 2021 at 03:42:23PM +0000, Stefan Hajnoczi wrote:
> On Mon, Dec 14, 2020 at 12:10:15PM +0100, Igor Mammedov wrote:
> > On Wed, 16 Sep 2020 10:51:49 +0100
> > Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > > Let -object memory-backend-file work on read-only files when the
> > > readonly=on option is given. This can be used to share the contents of a
> > > file between multiple guests while preventing them from consuming
> > > Copy-on-Write memory if guests dirty the pages, for example.
> > > 
> > > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > cosmetic/style nits only
> > 
> > s/Object *o/Object *obj/
> > 
> > for consistency with the rest of the code in file.
> 
> Will fix.
> 
> > > @@ -153,6 +154,26 @@ static void file_memory_backend_set_pmem(Object *o, bool value, Error **errp)
> > >      fb->is_pmem = value;
> > >  }
> > >  
> > > +static bool file_memory_backend_get_readonly(Object *o, Error **errp)
> > > +{
> > > +    return MEMORY_BACKEND_FILE(o)->readonly;
> > 
> > I thought using macro this way not acceptable and one should use
> > 
> > HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
> > 
> > return fb->readonly;
> 
> I'm not sure where this is forbidden or why? I've updated the patch as
> suggested anyway.

I have a vague memory of seeing this documented somewhere, but I
can't find it anywhere in the QOM documentation or git log.

I don't think we need to make this a rule, though.

-- 
Eduardo