[PATCH 4.9 61/78] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and_clear

From: Greg Kroah-Hartman
Date: Thu Sep 13 2018 - 09:38:52 EST

4.9-stable review patch. If anyone has any objections, please let me know.


From: Juergen Gross <jgross@xxxxxxxx>

commit b2d7a075a1ccef2fb321d595802190c8e9b39004 upstream.

Using only 32-bit writes for the pte will result in an intermediate
L1TF vulnerable PTE. When running as a Xen PV guest this will at once
switch the guest to shadow mode resulting in a loss of performance.

Use arch_atomic64_xchg() instead which will perform the requested
operation atomically with all 64 bits.

Some performance considerations according to:


The main number should be the latency, as there is no tight loop around

"lock cmpxchg8b" has a latency of 20 cycles, while "lock xchg" (with a
memory operand) isn't mentioned in that document. "lock xadd" (with xadd
having 3 cycles less latency than xchg) has a latency of 11, so we can
assume a latency of 14 for "lock xchg".

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Jason Andryuk <jandryuk@xxxxxxxxx>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/x86/include/asm/pgtable-3level.h | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

--- a/arch/x86/include/asm/pgtable-3level.h
+++ b/arch/x86/include/asm/pgtable-3level.h
@@ -1,6 +1,8 @@

+#include <asm/atomic64_32.h>
* Intel Physical Address Extension (PAE) Mode - three-level page
* tables on PPro+ CPUs.
@@ -142,10 +144,7 @@ static inline pte_t native_ptep_get_and_
pte_t res;

- /* xchg acts as a barrier before the setting of the high bits */
- res.pte_low = xchg(&ptep->pte_low, 0);
- res.pte_high = ptep->pte_high;
- ptep->pte_high = 0;
+ res.pte = (pteval_t)arch_atomic64_xchg((atomic64_t *)ptep, 0);

return res;