* Juergen Gross <jgross@xxxxxxxx> 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@xxxxxxxx>
---
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
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