Re: [Qemu-devel] [PATCH 8/9] Add memfd based hostmem

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 12 Apr 2016 13:55:28 +0200
marcandre.lureau@redhat.com wrote:

> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> Add a new memory backend, similar to hostmem-file, except that it
> doesn't need a file path, or hugepages directory permissions. It also
> try to enforce memory sealing if available.
> 
> This backend is mainly useful for easily sharing the memory with other
> processes, for example with vhost-user backend, when hugepages aren't
> available (for security reasons and other limitations).
> 
> Usage:
> -object memory-backend-memfd,id=mem1,size=1G
Does it make sense if it's possible to reuse/extend memory-backend-file,
i.e. make 'mem-path' optional and behave like this patch
and also add 'memfd' property so that parent process could
pass its handle to QEMU as well?

> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  backends/Makefile.objs   |  1 +
>  backends/hostmem-memfd.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++
>  qemu-options.hx          | 12 +++++++
>  3 files changed, 98 insertions(+)
>  create mode 100644 backends/hostmem-memfd.c
> 
> diff --git a/backends/Makefile.objs b/backends/Makefile.objs
> index 31a3a89..32cfcdb 100644
> --- a/backends/Makefile.objs
> +++ b/backends/Makefile.objs
> @@ -9,3 +9,4 @@ common-obj-$(CONFIG_TPM) += tpm.o
>  
>  common-obj-y += hostmem.o hostmem-ram.o
>  common-obj-$(CONFIG_LINUX) += hostmem-file.o
> +common-obj-$(CONFIG_POSIX) += hostmem-memfd.o
> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> new file mode 100644
> index 0000000..e0e18d7
> --- /dev/null
> +++ b/backends/hostmem-memfd.c
> @@ -0,0 +1,85 @@
> +/*
> + * QEMU host memfd memory backend
> + *
> + * Copyright (C) 2016 Red Hat Inc
> + *
> + * Authors:
> + *   Marc-André Lureau <marcandre.lureau@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +#include "qemu/osdep.h"
> +#include "qemu-common.h"
> +#include "sysemu/hostmem.h"
> +#include "sysemu/sysemu.h"
> +#include "qom/object_interfaces.h"
> +#include "qemu/memfd.h"
> +#include "qapi/error.h"
> +
> +/**
> + * @TYPE_MEMORY_BACKEND_MEMFD:
> + * name of the memory backend that uses memfd mmap
> + */
> +#define TYPE_MEMORY_BACKEND_MEMFD "memory-backend-memfd"
> +
> +#define MEMORY_BACKEND_MEMFD(obj) \
> +    OBJECT_CHECK(HostMemoryBackendMemfd, (obj), TYPE_MEMORY_BACKEND_MEMFD)
> +
> +typedef struct HostMemoryBackendMemfd HostMemoryBackendMemfd;
> +
> +struct HostMemoryBackendMemfd {
> +    HostMemoryBackend parent_obj;
> +
> +    int fd;
> +};
> +
> +static void
> +memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
> +{
> +    int fd;
> +
> +    if (!backend->size) {
> +        error_setg(errp, "can't create backend with size 0");
> +        return;
> +    }
> +#ifndef CONFIG_LINUX
> +    error_setg(errp, "memfd not supported on this host");
> +#else
> +    if (!memory_region_size(&backend->mr)) {
> +        backend->force_prealloc = mem_prealloc;
> +        fd = qemu_memfd_create(TYPE_MEMORY_BACKEND_MEMFD,
> +                               backend->size,
> +                               F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL);
> +        if (fd == -1) {
> +            error_setg(errp, "can't allocate memfd backend");
> +            return;
> +        }
> +        memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
> +            object_get_canonical_path(OBJECT(backend)),
> +            backend->size, true, fd, errp);
> +    }
> +#endif
> +}
> +
> +static void
> +memfd_backend_class_init(ObjectClass *oc, void *data)
> +{
> +    HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
> +
> +    bc->alloc = memfd_backend_memory_alloc;
> +}
> +
> +static const TypeInfo memfd_backend_info = {
> +    .name = TYPE_MEMORY_BACKEND_MEMFD,
> +    .parent = TYPE_MEMORY_BACKEND,
> +    .class_init = memfd_backend_class_init,
> +    .instance_size = sizeof(HostMemoryBackendMemfd),
> +};
> +
> +static void register_types(void)
> +{
> +    type_register_static(&memfd_backend_info);
> +}
> +
> +type_init(register_types);
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 587de8f..a54af0f 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3756,6 +3756,18 @@ The @option{share} boolean option determines whether the memory
>  region is marked as private to QEMU, or shared. The latter allows
>  a co-operating external process to access the QEMU memory region.
>  
> +@item -object memory-backend-memfd,id=@var{id},size=@var{size}
> +
> +Creates an anonymous memory file backend object, which can be used to
> +share the memory with a co-operating external process. The memory is
> +created with memfd and sealing if possible, but has fallback mechanism
> +for systems that lack it.
> +
> +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}.
> +
>  @item -object rng-random,id=@var{id},filename=@var{/dev/random}
>  
>  Creates a random number generator backend which obtains entropy from