xenvbd driver modifies in-flight data?

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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