[PATCH v2 2/3] x86/mm: Fix early boot use of INVPLGB

[Rik van Riel]

The INVLPGB instruction has limits on how many pages it can invalidate
at once. That limit is enumerated in CPUID, read by the kernel, and
stored in 'invpgb_count_max'. Ranged invalidation, like
invlpgb_kernel_range_flush() break up their invalidations so
that they do not exceed the limit.

However, early boot code currently attempts to do ranged
invalidation before populating 'invlpgb_count_max'. There is a
for loop which is basically:

	for (...; addr < end; addr += invlpgb_count_max*PAGE_SIZE)

If invlpgb_kernel_range_flush is called before the kernel has read
the value of invlpgb_count_max from the hardware, the normally
bounded loop can become an infinite loop if invlpgb_count_max is
initialized to zero.

Fix that issue by initializing invlpgb_count_max to 1.

This way INVPLGB at early boot time will be a little bit slower
than normal (with initialized invplgb_count_max), and not an
instant hang at bootup time.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Fixes: b7aa05cbdc52 ("x86/mm: Add INVLPGB support code")
Cc: stable@xxxxxxxxxx
---
 arch/x86/kernel/cpu/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 93da466dfe2c..b2ad8d13211a 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -31,7 +31,7 @@
 
 #include "cpu.h"
 
-u16 invlpgb_count_max __ro_after_init;
+u16 invlpgb_count_max __ro_after_init = 1;
 
 static inline int rdmsrq_amd_safe(unsigned msr, u64 *p)
 {
-- 
2.49.0