Re: [Qemu-devel] [PATCH v4 2/2] vhost-user: Attempt to fix a race with set_mem_table.

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 27, 2016 at 02:52:37AM -0700, Prerna Saxena wrote:
> From: Prerna Saxena <prerna.saxena@nutanix.com>
> 
> The set_mem_table command currently does not seek a reply. Hence, there is
> no easy way for a remote application to notify to QEMU when it finished
> setting up memory, or if there were errors doing so.
> 
> As an example:
> (1) Qemu sends a SET_MEM_TABLE to the backend (eg, a vhost-user net
> application). SET_MEM_TABLE does not require a reply according to the spec.
> (2) Qemu commits the memory to the guest.
> (3) Guest issues an I/O operation over a new memory region which was configured on (1).
> (4) The application has not yet remapped the memory, but it sees the I/O request.
> (5) The application cannot satisfy the request because it does not know about those GPAs.
> 
> While a guaranteed fix would require a protocol extension (committed separately),
> a best-effort workaround for existing applications is to send a GET_FEATURES
> message before completing the vhost_user_set_mem_table() call.
> Since GET_FEATURES requires a reply, an application that processes vhost-user
> messages synchronously would probably have completed the SET_MEM_TABLE before replying.
> 
> Signed-off-by: Prerna Saxena <prerna.saxena@nutanix.com>

Could you pls reorder patchset so this is 1/2?
1/1 is still under review but I'd like to make sure
we have some kind of fix in place for 2.7.

> ---
>  hw/virtio/vhost-user.c | 123 ++++++++++++++++++++++++++-----------------------
>  1 file changed, 65 insertions(+), 58 deletions(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 0cdb918..f96607e 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -254,64 +254,6 @@ static int vhost_user_set_log_base(struct vhost_dev *dev, uint64_t base,
>      return 0;
>  }
>  
> -static int vhost_user_set_mem_table(struct vhost_dev *dev,
> -                                    struct vhost_memory *mem)
> -{
> -    int fds[VHOST_MEMORY_MAX_NREGIONS];
> -    int i, fd;
> -    size_t fd_num = 0;
> -    bool reply_supported = virtio_has_feature(dev->protocol_features,
> -                            VHOST_USER_PROTOCOL_F_REPLY_ACK);
> -
> -    VhostUserMsg msg = {
> -        .request = VHOST_USER_SET_MEM_TABLE,
> -        .flags = VHOST_USER_VERSION,
> -    };
> -
> -    if (reply_supported) {
> -        msg.flags |= VHOST_USER_NEED_RESPONSE_MASK;
> -    }
> -
> -    for (i = 0; i < dev->mem->nregions; ++i) {
> -        struct vhost_memory_region *reg = dev->mem->regions + i;
> -        ram_addr_t offset;
> -        MemoryRegion *mr;
> -
> -        assert((uintptr_t)reg->userspace_addr == reg->userspace_addr);
> -        mr = memory_region_from_host((void *)(uintptr_t)reg->userspace_addr,
> -                                     &offset);
> -        fd = memory_region_get_fd(mr);
> -        if (fd > 0) {
> -            msg.payload.memory.regions[fd_num].userspace_addr = reg->userspace_addr;
> -            msg.payload.memory.regions[fd_num].memory_size  = reg->memory_size;
> -            msg.payload.memory.regions[fd_num].guest_phys_addr = reg->guest_phys_addr;
> -            msg.payload.memory.regions[fd_num].mmap_offset = offset;
> -            assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
> -            fds[fd_num++] = fd;
> -        }
> -    }
> -
> -    msg.payload.memory.nregions = fd_num;
> -
> -    if (!fd_num) {
> -        error_report("Failed initializing vhost-user memory map, "
> -                     "consider using -object memory-backend-file share=on");
> -        return -1;
> -    }
> -
> -    msg.size = sizeof(msg.payload.memory.nregions);
> -    msg.size += sizeof(msg.payload.memory.padding);
> -    msg.size += fd_num * sizeof(VhostUserMemoryRegion);
> -
> -    vhost_user_write(dev, &msg, fds, fd_num);
> -
> -    if (reply_supported) {
> -        return process_message_reply(dev, msg.request);
> -    }
> -
> -    return 0;
> -}
> -
>  static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>                                       struct vhost_vring_addr *addr)
>  {
> @@ -514,6 +456,71 @@ static int vhost_user_get_features(struct vhost_dev *dev, uint64_t *features)
>      return vhost_user_get_u64(dev, VHOST_USER_GET_FEATURES, features);
>  }
>  
> +static int vhost_user_set_mem_table(struct vhost_dev *dev,
> +                                    struct vhost_memory *mem)
> +{
> +    int fds[VHOST_MEMORY_MAX_NREGIONS];
> +    int i, fd;
> +    size_t fd_num = 0;
> +    uint64_t features;
> +    bool reply_supported = virtio_has_feature(dev->protocol_features,
> +                            VHOST_USER_PROTOCOL_F_REPLY_ACK);
> +
> +    VhostUserMsg msg = {
> +        .request = VHOST_USER_SET_MEM_TABLE,
> +        .flags = VHOST_USER_VERSION,
> +    };
> +
> +    if (reply_supported) {
> +        msg.flags |= VHOST_USER_NEED_RESPONSE_MASK;
> +    }
> +
> +    for (i = 0; i < dev->mem->nregions; ++i) {
> +        struct vhost_memory_region *reg = dev->mem->regions + i;
> +        ram_addr_t offset;
> +        MemoryRegion *mr;
> +
> +        assert((uintptr_t)reg->userspace_addr == reg->userspace_addr);
> +        mr = memory_region_from_host((void *)(uintptr_t)reg->userspace_addr,
> +                                     &offset);
> +        fd = memory_region_get_fd(mr);
> +        if (fd > 0) {
> +            msg.payload.memory.regions[fd_num].userspace_addr = reg->userspace_addr;
> +            msg.payload.memory.regions[fd_num].memory_size  = reg->memory_size;
> +            msg.payload.memory.regions[fd_num].guest_phys_addr = reg->guest_phys_addr;
> +            msg.payload.memory.regions[fd_num].mmap_offset = offset;
> +            assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
> +            fds[fd_num++] = fd;
> +        }
> +    }
> +
> +    msg.payload.memory.nregions = fd_num;
> +
> +    if (!fd_num) {
> +        error_report("Failed initializing vhost-user memory map, "
> +                     "consider using -object memory-backend-file share=on");
> +        return -1;
> +    }
> +
> +    msg.size = sizeof(msg.payload.memory.nregions);
> +    msg.size += sizeof(msg.payload.memory.padding);
> +    msg.size += fd_num * sizeof(VhostUserMemoryRegion);
> +
> +    vhost_user_write(dev, &msg, fds, fd_num);
> +
> +    if (reply_supported) {
> +        return process_message_reply(dev, msg.request);
> +    } else {
> +        /* Note: It is (yet) unknown when the client application has finished
> +         * remapping the GPA.
> +         * Attempt to prevent a race by sending a command that requires a reply.
> +         */
> +        vhost_user_get_features(dev, &features);
> +    }
> +
> +    return 0;
> +}
> +
>  static int vhost_user_set_owner(struct vhost_dev *dev)
>  {
>      VhostUserMsg msg = {
> -- 
> 1.8.1.2