Re: [PATCH 1/2] virtio: use virtio accessor to access packed descriptor flags

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

On Thu, Nov 11, 2021 at 02:38:53PM +0800, Jason Wang wrote:
> We used to access packed descriptor flags via
> address_space_{write|read}_cached(). When we hit the cache, memcpy()
> is used which is not an atomic operation which may lead a wrong value
> is read or wrote.

Could you clarify where's the memcpy that you see?
Thanks!

> So this patch switches to use virito_{stw|lduw}_phys_cached() to make
> sure the aceess is atomic.
> 
> Fixes: 86044b24e865f ("virtio: basic packed virtqueue support")
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/virtio/virtio.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index cc69a9b881..939bcbfeb9 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -507,11 +507,9 @@ static void vring_packed_desc_read_flags(VirtIODevice *vdev,
>                                           MemoryRegionCache *cache,
>                                           int i)
>  {
> -    address_space_read_cached(cache,
> -                              i * sizeof(VRingPackedDesc) +
> -                              offsetof(VRingPackedDesc, flags),
> -                              flags, sizeof(*flags));
> -    virtio_tswap16s(vdev, flags);
> +    hwaddr off = i * sizeof(VRingPackedDesc) + offsetof(VRingPackedDesc, flags);
> +
> +    *flags = virtio_lduw_phys_cached(vdev, cache, off);
>  }
>  
>  static void vring_packed_desc_read(VirtIODevice *vdev,
> @@ -564,8 +562,7 @@ static void vring_packed_desc_write_flags(VirtIODevice *vdev,
>  {
>      hwaddr off = i * sizeof(VRingPackedDesc) + offsetof(VRingPackedDesc, flags);
>  
> -    virtio_tswap16s(vdev, &desc->flags);
> -    address_space_write_cached(cache, off, &desc->flags, sizeof(desc->flags));
> +    virtio_stw_phys_cached(vdev, cache, off, desc->flags);
>      address_space_cache_invalidate(cache, off, sizeof(desc->flags));
>  }
>  
> -- 
> 2.25.1