Xen PAT settings vs Linux PAT settings

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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Hi,

It looks like we've identified the second buggy driver that somewhere
assumes PAT is configured as Linux normally do natively - nvidia binary
one this time[3]. The first one affected was i915, but it turned out to be
a bug in Linux mm. It was eventually fixed[1], but it was quite painful
debugging. This time a proper fix is not known yet. Since the previous
issue, Qubes OS carried a patch[2] that changes Xen to use same PAT as
Linux. We recently dropped this patch, since the Linux fix reached all
supported by us branches, but apparently it wasn't all...

Anyway, would it be useful (and acceptable) for upstream Xen to have
a kconfig option (behind UNSUPPORTED or so) to switch this behavior?
Technically, it's a PV ABI violation, and it does break few things
(definitely PV domU with passthrough are affected - Xen considers them
L1TF vulnerable then; PV live migration is most likely broken too). But
on the other hand, if one doesn't use affected feature, it allows to
workaround an issue that otherwise is very annoying to debug...


[1] git.kernel.org/torvalds/c/548cb932051fb6232ac983ed6673dae7bdf3cf4c
[2] https://github.com/QubesOS/qubes-vmm-xen/blob/44e9fd9f3b1ebf1cf43674b5a1c2669f7dd253f5/1019-Use-Linux-s-PAT.patch
[3] https://github.com/QubesOS/qubes-issues/issues/9501
-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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