Re: xenvbd driver modifies in-flight data?

["Durrant, Paul" <xadimgnik@xxxxxxxxx>]

On 04/10/2024 14:07, Marek Marczykowski-Górecki wrote:
> Hi,
>
> I've got a bit worrying report[1], that using Windows PV block driver,
> together with backing file opened with O_DIRECT on BTRFS results in
> checksum errors. At this point it's unclear what exactly happens there,
> it could be some BTRFS issue, some xen-blkback issue, but since
> according to reports it happens only with Windows DomU, the most likely
> culprit is PV driver inside Windows DomU. It was detected on BTRFS
> because it does checksumming, but it isn't clear whether other file
> systems are not affected too.
>
> I guess using O_DIRECT is asking for troubles here, but nevertheless, it
> looks like it might have uncovered some issue that would be silently
> ignored otherwise.
>
> [1] https://github.com/QubesOS/qubes-issues/issues/9488#issuecomment-2389152014
Using O_DIRECT is the norm for Xen block backends to avoid anything 
being cached. I've never heard of XENVBD ever causing corruption 
either... The only corruption cases I ever came across were because 
something had gone wrong with emulated unplug and so the IDE device was 
still present in the VM and was treated as a separate disk.
  Cheers,

    Paul