[PATCH v4] x86/mtrr: Check if fixed MTRRs exist before saving them

[Andi Kleen]

MTRRs have a obsolete fixed variant for fine grained
caching control of the 640K-1MB region. This fixed variant has a
separate capability bit in the MTRR capability MSR. Most of the MTRR code
checks this capability bit before trying to access the fixed MTRR MSRs,
except in one place. This patch fixes this place to also
check the capability.

Otherwise there will be a WARN_ON due to the #GP when the respective MSRs
are accessed to save them.

Fixes: 2b1f6278d77c ("[PATCH] x86: Save the MTRRs of the BSP before booting an AP")
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

---

v2: Add Fixes tag and expand description.
v3: Expand description
v4: Expand description
---
 arch/x86/kernel/cpu/mtrr/mtrr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
index 767bf1c71aad..2a2fc14955cd 100644
--- a/arch/x86/kernel/cpu/mtrr/mtrr.c
+++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
@@ -609,7 +609,7 @@ void mtrr_save_state(void)
 {
 	int first_cpu;
 
-	if (!mtrr_enabled())
+	if (!mtrr_enabled() || !mtrr_state.have_fixed)
 		return;
 
 	first_cpu = cpumask_first(cpu_online_mask);
-- 
2.45.1