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    SubjectRe: [PATCH 2/6] x86/pat: check for MTRRs enabled in memtype_reserve()

    * 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.

    Also, speling nit:

    s/one of both
    /one or both

    Thanks,

    Ingo

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