Re: Xen PAT settings vs Linux PAT settings

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

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On Mon, Oct 14, 2024 at 09:05:58PM +0100, Andrew Cooper wrote:
> On 14/10/2024 7:26 pm, Marek Marczykowski-Górecki wrote:
> > 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?
> 
> Not UNSUPPORTED - it's bogus and I still want it purged.
> 
> But, behind EXPERT, with a suitable description (e.g. "This breaks
> various ABIs including migration, and is presented here for debugging PV
> driver issues in a single system.  If turning it on fixes a bug, please
> contact upstream Xen"), then I think we need to take it.

Makes sense.

> The fact that I've had to recommend it once already this week for
> debugging purposes, and it wasn't even this Nvidia bug, demonstrates how
> pervasive the problems are.
> 
> > 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).
> 
> Do you have more information on this?  The PAT bits shouldn't form any
> part of L1TF considerations.

https://github.com/QubesOS/qubes-issues/issues/8593

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

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