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micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="oh/fywFlm7BR8i0T" Content-Disposition: inline In-Reply-To: --oh/fywFlm7BR8i0T Content-Type: text/plain; protected-headers=v1; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Date: Mon, 14 Oct 2024 23:37:13 +0200 From: Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= To: Andrew Cooper Cc: xen-devel Subject: Re: Xen PAT settings vs Linux PAT settings On Mon, Oct 14, 2024 at 09:05:58PM +0100, Andrew Cooper wrote: > On 14/10/2024 7:26 pm, Marek Marczykowski-G=C3=B3recki 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? >=20 > Not UNSUPPORTED - it's bogus and I still want it purged. >=20 > 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.=C2=A0 If turning it on fixes a bug, ple= ase > 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. >=20 > > 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). >=20 > Do you have more information on this?=C2=A0 The PAT bits shouldn't form a= ny > part of L1TF considerations. https://github.com/QubesOS/qubes-issues/issues/8593 --=20 Best Regards, Marek Marczykowski-G=C3=B3recki Invisible Things Lab --oh/fywFlm7BR8i0T Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhrpukzGPukRmQqkK24/THMrX1ywFAmcNjwkACgkQ24/THMrX 1yzfQwgAh/956Z8NeyGlNS6LADzp9jI8gLUJ1rx8ZA+VczGrI8/KPXX47xSuqiS4 Vlmmr32nyusZKIcRzKw4CiQrs5UZK/9IeVTqaJM5fsjCPHlnK5gpUwHV3XtaCdoN VRGnNhLB18gXpgGhwb5jnaCWaSa68kcvrIqJZ8z779ySmTPKQecxdY7M5pGgCXRX znZnjizJelzt2Dqr9Ty0FE8gcYzU8nk7ic2pTL51E+hsIbqH/x5ZJRf5PKLvCZNU 0aWXHg/I81o0fjRt5u0BbS3YkVfqHXhU74qFjHfxnVxHzm2PbpuGcGbZxKD8F8zu masCUcNqzzoFQUAkykwazIItkfJ6GA== =YciZ -----END PGP SIGNATURE----- --oh/fywFlm7BR8i0T--