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

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

On Fri, Nov 12, 2021 at 10:23:12AM +0800, Jason Wang wrote:
> 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

But that's a copy from the cache, not from guest memory.
I don't see how it can change so I don't see why it needs
to be atomic. Was there an actual issue you observed or
is this theoretical?


> >
> > > 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
> >