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

[Juergen Gross]

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;
 		return 0;
@@ -627,7 +631,7 @@ int memtype_free(u64 start, u64 end)
 	int is_range_ram;
 	struct memtype *entry_old;
 
-	if (!pat_enabled())
+	if (!pat_enabled() || !mtrr_enabled())
 		return 0;
 
 	start = sanitize_phys(start);
-- 
2.35.3