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

[Jason Wang <jasowang@redhat.com>]

On Thu, Nov 11, 2021 at 4:27 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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!

In the address_space_{write|read}_cached it self:

static inline MemTxResult
=>dress_space_write_cached(MemoryRegionCache *cache, hwaddr addr,
                           const void *buf, hwaddr len)
{
    assert(addr < cache->len && len <= cache->len - addr);
    if (likely(cache->ptr)) {
        memcpy(cache->ptr + addr, buf, len);
        return MEMTX_OK;
    } else {
        return address_space_write_cached_slow(cache, addr, buf, len);
    }
}

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
>