Re: [PATCH 1/1] hv: drivers: vmbus: Prevent load re-ordering when reading ring buffer

Andrea Parri
2022-03-28T23:12:46Z

On Sun, Mar 27, 2022 at 08:25:10AM -0700, Michael Kelley wrote:
> When reading a packet from a host-to-guest ring buffer, there is no
> memory barrier between reading the write index (to see if there is
> a packet to read) and reading the contents of the packet. The Hyper-V
> host uses store-release when updating the write index to ensure that
> writes of the packet data are completed first. On the guest side,
> the processor can reorder and read the packet data before the write
> index, and sometimes get stale packet data. Getting such stale packet
> data has been observed in a reproducible case in a VM on ARM64.
>
> Fix this by using virt_load_acquire() to read the write index,
> ensuring that reads of the packet data cannot be reordered
> before it. Preventing such reordering is logically correct, and
> with this change, getting stale data can no longer be reproduced.
>
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>

Nit: subject prefix -> "Drivers: hv: vmbus:".

Thanks,
Andrea