Messages in this thread | | | Date | Tue, 7 Feb 2023 10:12:27 +0100 | From | Juergen Gross <> | Subject | Re: [PATCH 2/6] x86/pat: check for MTRRs enabled in memtype_reserve() |
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On 07.02.23 09:49, Ingo Molnar wrote: > > * Juergen Gross <jgross@suse.com> wrote: > >> Today memtype_reserve() bails out early if pat_enabled() returns false. >> The same can be done in case MTRRs aren't enabled. >> >> This will reinstate the behavior of memtype_reserve() before commit >> 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when >> running on Xen"). There have been reports about that commit breaking >> SEV-SNP guests under Hyper-V, which was tried to be resolved by commit >> 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case"), >> but that again resulted in problems with Xen PV guests. >> >> Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen") >> Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case") >> Signed-off-by: Juergen Gross <jgross@suse.com> >> --- >> arch/x86/mm/pat/memtype.c | 10 +++++++--- >> 1 file changed, 7 insertions(+), 3 deletions(-) >> >> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c >> index fb4b1b5e0dea..18f612b43763 100644 >> --- a/arch/x86/mm/pat/memtype.c >> +++ b/arch/x86/mm/pat/memtype.c >> @@ -557,8 +557,12 @@ int memtype_reserve(u64 start, u64 end, enum page_cache_mode req_type, >> return -EINVAL; >> } >> >> - if (!pat_enabled()) { >> - /* This is identical to page table setting without PAT */ >> + /* >> + * PAT disabled or MTRRs disabled don't require any memory type >> + * tracking or type adjustments, as there can't be any conflicts >> + * between PAT and MTRRs with at least one of both being disabled. >> + */ >> + if (!pat_enabled() || !mtrr_enabled()) { >> if (new_type) >> *new_type = req_type; > > Doesn't memtype_reserve() also check for overlapping ranges & type > compatibility in memtype_check_conflict(), etc., which can occur even in a > pure PAT setup? Ie. are we 100% sure that in the !MTRR case it would be a > NOP? > > But even if it's a functional NOP as you claim, we'd still be better off if > the memtype tree was still intact - instead of just turning off the API.
Yes, that's basically the issue discussed in [patch 0/6].
It should still be better than the original case (PAT and MTRR off, but the ability to use PAT nevertheless), though.
> > Also, speling nit: > > s/one of both > /one or both
Hmm, but only if I drop the "at least". I don't really mind either way.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |