Re: [Qemu-devel] [PATCH v3 1/3] hostmem-file: add "align" option

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Nov 27, 2017 at 12:35:15PM +0800, Haozhong Zhang wrote:
> When mmap(2) the backend files, QEMU uses the host page size
> (getpagesize(2)) by default as the alignment of mapping address.
> However, some backends may require alignments different than the page
> size. For example, mmap a device DAX (e.g., /dev/dax0.0) on Linux
> kernel 4.13 to an address, which is 4K-aligned but not 2M-aligned,
> fails with a kernel message like
> 
> [617494.969768] dax dax0.0: qemu-system-x86: dax_mmap: fail, unaligned vma (0x7fa37c579000 - 0x7fa43c579000, 0x1fffff)
> 
> Because there is no common approach to get such alignment requirement,
> we add the 'align' option to 'memory-backend-file', so that users or
> management utils, which have enough knowledge about the backend, can
> specify a proper alignment via this option.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>

The new option needs to be documented on qemu-options.hx.

Except for that, the patch looks good to me.


> ---
>  backends/hostmem-file.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  docs/nvdimm.txt         | 16 ++++++++++++++++
>  exec.c                  |  8 +++++++-
>  include/exec/memory.h   |  3 +++
>  memory.c                |  2 ++
>  numa.c                  |  2 +-
>  6 files changed, 69 insertions(+), 3 deletions(-)
> 
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index e44c319915..e319ec1ad8 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -34,6 +34,7 @@ struct HostMemoryBackendFile {
>      bool share;
>      bool discard_data;
>      char *mem_path;
> +    uint64_t align;
>  };
>  
>  static void
> @@ -58,7 +59,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>          path = object_get_canonical_path(OBJECT(backend));
>          memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
>                                   path,
> -                                 backend->size, fb->share,
> +                                 backend->size, fb->align, fb->share,
>                                   fb->mem_path, errp);
>          g_free(path);
>      }
> @@ -115,6 +116,40 @@ static void file_memory_backend_set_discard_data(Object *o, bool value,
>      MEMORY_BACKEND_FILE(o)->discard_data = value;
>  }
>  
> +static void file_memory_backend_get_align(Object *o, Visitor *v,
> +                                          const char *name, void *opaque,
> +                                          Error **errp)
> +{
> +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
> +    uint64_t val = fb->align;
> +
> +    visit_type_size(v, name, &val, errp);
> +}
> +
> +static void file_memory_backend_set_align(Object *o, Visitor *v,
> +                                          const char *name, void *opaque,
> +                                          Error **errp)
> +{
> +    HostMemoryBackend *backend = MEMORY_BACKEND(o);
> +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
> +    Error *local_err = NULL;
> +    uint64_t val;
> +
> +    if (host_memory_backend_mr_inited(backend)) {
> +        error_setg(&local_err, "cannot change property value");
> +        goto out;
> +    }
> +
> +    visit_type_size(v, name, &val, &local_err);
> +    if (local_err) {
> +        goto out;
> +    }
> +    fb->align = val;
> +
> + out:
> +    error_propagate(errp, local_err);
> +}
> +
>  static void file_backend_unparent(Object *obj)
>  {
>      HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> @@ -145,6 +180,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
>      object_class_property_add_str(oc, "mem-path",
>          get_mem_path, set_mem_path,
>          &error_abort);
> +    object_class_property_add(oc, "align", "int",
> +        file_memory_backend_get_align,
> +        file_memory_backend_set_align,
> +        NULL, NULL, &error_abort);
>  }
>  
>  static void file_backend_instance_finalize(Object *o)
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index 2d9f8c0e8c..21249dd062 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -122,3 +122,19 @@ Note:
>       M >= size of RAM devices +
>            size of statically plugged vNVDIMM devices +
>            size of hotplugged vNVDIMM devices
> +
> +Alignment
> +---------
> +
> +QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping
> +address to the page size (getpagesize(2)) by default. However, some
> +types of backends may require an alignment different than the page
> +size. In that case, QEMU v2.12.0 and later provide 'align' option to
> +memory-backend-file to allow users to specify the proper alignment.
> +
> +For example, device dax require the 2 MB alignment, so we can use
> +following QEMU command line options to use it (/dev/dax0.0) as the
> +backend of vNVDIMM:
> +
> + -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M
> + -device nvdimm,id=nvdimm1,memdev=mem1
> diff --git a/exec.c b/exec.c
> index 03238a3449..90440efecd 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1600,7 +1600,13 @@ static void *file_ram_alloc(RAMBlock *block,
>      void *area;
>  
>      block->page_size = qemu_fd_getpagesize(fd);
> -    block->mr->align = block->page_size;
> +    if (block->mr->align % block->page_size) {
> +        error_setg(errp, "aligment 0x%" PRIx64
> +                   " must be multiples of page size 0x%" PRIx64,
> +                   block->mr->align, block->page_size);
> +        return NULL;
> +    }
> +    block->mr->align = MAX(block->page_size, block->mr->align);
>  #if defined(__s390x__)
>      if (kvm_enabled()) {
>          block->mr->align = MAX(block->mr->align, QEMU_VMALLOC_ALIGN);
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 5ed4042f87..a1be8b06d3 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -465,6 +465,8 @@ void memory_region_init_resizeable_ram(MemoryRegion *mr,
>   * @name: Region name, becomes part of RAMBlock name used in migration stream
>   *        must be unique within any device
>   * @size: size of the region.
> + * @align: alignment of the region base address; if 0, the default alignment
> + *         (getpagesize()) will be used.
>   * @share: %true if memory must be mmaped with the MAP_SHARED flag
>   * @path: the path in which to allocate the RAM.
>   * @errp: pointer to Error*, to store an error if it happens.
> @@ -476,6 +478,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>                                        struct Object *owner,
>                                        const char *name,
>                                        uint64_t size,
> +                                      uint64_t align,
>                                        bool share,
>                                        const char *path,
>                                        Error **errp);
> diff --git a/memory.c b/memory.c
> index e26e5a3b1d..2c80656e39 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -1570,6 +1570,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>                                        struct Object *owner,
>                                        const char *name,
>                                        uint64_t size,
> +                                      uint64_t align,
>                                        bool share,
>                                        const char *path,
>                                        Error **errp)
> @@ -1578,6 +1579,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
>      mr->ram = true;
>      mr->terminates = true;
>      mr->destructor = memory_region_destructor_ram;
> +    mr->align = align;
>      mr->ram_block = qemu_ram_alloc_from_file(size, mr, share, path, errp);
>      mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>  }
> diff --git a/numa.c b/numa.c
> index 7151b24d1c..b0fe22a60c 100644
> --- a/numa.c
> +++ b/numa.c
> @@ -551,7 +551,7 @@ static void allocate_system_memory_nonnuma(MemoryRegion *mr, Object *owner,
>      if (mem_path) {
>  #ifdef __linux__
>          Error *err = NULL;
> -        memory_region_init_ram_from_file(mr, owner, name, ram_size, false,
> +        memory_region_init_ram_from_file(mr, owner, name, ram_size, 0, false,
>                                           mem_path, &err);
>          if (err) {
>              error_report_err(err);
> -- 
> 2.14.1
> 

-- 
Eduardo