[PATCH v5] x86/mtrr: Check if fixed MTRRs exist before saving them

[Andi Kleen]

MTRRs have an obsolete fixed variant for fine grained caching control
of the 640K-1MB region that uses separate MSRs. This fixed variant has
a separate capability bit in the MTRR capability MSR, however all x86
CPUs which have MTRR generally had this separate bit set.

The code to save MTRRs forgets to check the capability bit before
accessing the fixed MTRR MSRs.  This results in a #GP on a system that
does not support the fixed MTRR capability. The #GP is harmless because
the RDMSR fault is handled and harmless, but results in a WARN_ON printout
that looks scary to the user.

Found by testing Linux on a setup without fixed MTRRs.

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
v5: Change 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);
-- 
2.45.2