Re: [Qemu-devel] [PATCH v5 5/6] Add memfd based hostmem

[Daniel P. Berrange]

On Mon, Oct 23, 2017 at 10:59:09AM +0100, Marc-André Lureau wrote:
> Add a new memory backend, similar to hostmem-file, except that it
> doesn't need to create files. It also enforces memory sealing.
> 
> This backend is mainly useful for sharing the memory with other
> processes.
> 
> Note that Linux supports transparent huge-pages of shmem/memfd memory
> since 4.8. It is relatively easier to set up THP than a dedicate
> hugepage mount point by using "madvise" in
> /sys/kernel/mm/transparent_hugepage/shmem_enabled.
> 
> Since 4.14, memfd allows to set hugetlb requirement explicitly.
> 
> Usage:
> -object memory-backend-memfd,id=mem1,size=1G

[snip]

> +static void
> +memfd_backend_class_init(ObjectClass *oc, void *data)
> +{
> +    HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
> +
> +    bc->alloc = memfd_backend_memory_alloc;
> +
> +    object_class_property_add_bool(oc, "hugetlb",
> +                                   memfd_backend_get_hugetlb,
> +                                   memfd_backend_set_hugetlb,
> +                                   &error_abort);

I tend to think that instead of a bool hugetlb, we should take an
integer page size instead eg  hugepagesize=2M instead of hugetlb=true



> diff --git a/qemu-options.hx b/qemu-options.hx
> index 3728e9b4dd..5828caefeb 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4208,6 +4208,21 @@ that @option{discard-data} is only an optimization, 
> and QEMU
>  might not discard file contents if it aborts unexpectedly or is
>  terminated using SIGKILL.
>  
> address@hidden -object 
> memory-backend-memfd,address@hidden,address@hidden,address@hidden|off},address@hidden|off}
> +
> +Creates an anonymous memory file backend object, which allows QEMU to
> +share the memory with an external process in some cases (e.g. when
> +using vhost-user). The memory is allocated with memfd and optional
> +sealing. (Linux only)
> +
> +The @option{id} parameter is a unique ID that will be used to
> +reference this memory region when configuring the @option{-numa}
> +argument. The @option{size} option provides the size of the memory
> +region, and accepts common suffixes, eg @option{500M}. The
> address@hidden option creates a sealed-file, that will block further
> +resizing the memory ('on' by default). The @option{hugetlb} option
> +specify the file to be created resides in the hugetlbfs filesystem.

This should document that you can't combine sealing and huge pages

Regards,
Daniel
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