Re: [PATCH 1/7] backends/hostmem: Drop redundant validation of property "size"

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Nov 11, 2026 at 01:23:36PM +0100, Markus Armbruster wrote:
> Property "size" is mandatory, and its setter
> host_memory_backend_set_size() rejects value zero.  Delete the
> redundant checks for zero in the .alloc() callbacks.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  backends/hostmem-epc.c   | 5 -----
>  backends/hostmem-file.c  | 4 ----
>  backends/hostmem-memfd.c | 5 -----
>  backends/hostmem-ram.c   | 5 -----
>  backends/hostmem-shm.c   | 5 -----
>  5 files changed, 24 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

...it is a shame this was/is dead code, since the actual error message
from host_memory_backend_set_size() is (IMHO) worse than that from the
unreachable code you're deleting

  $ ./build/qemu-system-x86_64 -object memory-backend-ram,size=0,id=foo
  qemu-system-x86_64: property 'size' of memory-backend-ram doesn't take value '0'

as a separate patch I might suggest

    diff --git a/backends/hostmem.c b/backends/hostmem.c
    index 173d042be35..f523dce6eb0 100644
    --- a/backends/hostmem.c
    +++ b/backends/hostmem.c
    @@ -77,8 +77,8 @@ host_memory_backend_set_size(Object *obj, Visitor *v, const char *name,
         }
         if (!value) {
             error_setg(errp,
    -                   "property '%s' of %s doesn't take value '%" PRIu64 "'",
    -                   name, object_get_typename(obj), value);
    +                   "cannot create '%s' with a size of 0",
    +                   object_get_typename(obj));
             return;
         }
         backend->size = value;

so that we get the nicer message:

    $ ./build/qemu-system-x86_64 -object memory-backend-ram,size=0,id=foo
    qemu-system-x86_64: cannot create 'memory-backend-ram' with a size of 0


> 
> diff --git a/backends/hostmem-epc.c b/backends/hostmem-epc.c
> index ab20b182331..e1c784249ff 100644
> --- a/backends/hostmem-epc.c
> +++ b/backends/hostmem-epc.c
> @@ -24,11 +24,6 @@ sgx_epc_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>      uint32_t ram_flags;
>      int fd;
>  
> -    if (!backend->size) {
> -        error_setg(errp, "can't create backend with size 0");
> -        return false;
> -    }
> -
>      fd = qemu_open("/dev/sgx_vepc", O_RDWR, errp);
>      if (fd < 0) {
>          return false;
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index 6ad9cad0320..08d0382d76f 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -48,10 +48,6 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>      g_autofree gchar *name = NULL;
>      uint32_t ram_flags;
>  
> -    if (!backend->size) {
> -        error_setg(errp, "can't create backend with size 0");
> -        return false;
> -    }
>      if (!fb->mem_path) {
>          error_setg(errp, "mem-path property not set");
>          return false;
> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> index 46489212a23..11d456e3ebd 100644
> --- a/backends/hostmem-memfd.c
> +++ b/backends/hostmem-memfd.c
> @@ -49,11 +49,6 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>      int fd = cpr_find_fd(name, 0);
>      uint32_t ram_flags;
>  
> -    if (!backend->size) {
> -        error_setg(errp, "can't create backend with size 0");
> -        return false;
> -    }
> -
>      if (fd >= 0) {
>          goto have_fd;
>      }
> diff --git a/backends/hostmem-ram.c b/backends/hostmem-ram.c
> index 6a507fad77c..0ae7b4ae759 100644
> --- a/backends/hostmem-ram.c
> +++ b/backends/hostmem-ram.c
> @@ -22,11 +22,6 @@ ram_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>      g_autofree char *name = NULL;
>      uint32_t ram_flags;
>  
> -    if (!backend->size) {
> -        error_setg(errp, "can't create backend with size 0");
> -        return false;
> -    }
> -
>      name = host_memory_backend_get_name(backend);
>      ram_flags = backend->share ? RAM_SHARED : RAM_PRIVATE;
>      ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
> diff --git a/backends/hostmem-shm.c b/backends/hostmem-shm.c
> index adf60957c3c..0a01c760551 100644
> --- a/backends/hostmem-shm.c
> +++ b/backends/hostmem-shm.c
> @@ -30,11 +30,6 @@ shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>      uint32_t ram_flags;
>      int fd = cpr_find_fd(backend_name, 0);
>  
> -    if (!backend->size) {
> -        error_setg(errp, "can't create shm backend with size 0");
> -        return false;
> -    }
> -
>      if (!backend->share) {
>          error_setg(errp, "can't create shm backend with `share=off`");
>          return false;
> -- 
> 2.55.0
> 

With regards,
Daniel
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