From b2d7a075a1ccef2fb321d595802190c8e9b39004 Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Tue, 21 Aug 2018 17:37:55 +0200 Subject: x86/pae: use 64 bit atomic xchg function in native_ptep_get_and_clear 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: https://software.intel.com/sites/default/files/managed/ad/dc/Intel-Xeon-Scalable-Processor-throughput-latency.pdf The main number should be the latency, as there is no tight loop around native_ptep_get_and_clear(). "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 Reviewed-by: Thomas Gleixner Reviewed-by: Jan Beulich Tested-by: Jason Andryuk Signed-off-by: Boris Ostrovsky --- arch/x86/include/asm/pgtable-3level.h | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/pgtable-3level.h b/arch/x86/include/asm/pgtable-3level.h index a564084c6141..f8b1ad2c3828 100644 --- a/arch/x86/include/asm/pgtable-3level.h +++ b/arch/x86/include/asm/pgtable-3level.h @@ -2,6 +2,8 @@ #ifndef _ASM_X86_PGTABLE_3LEVEL_H #define _ASM_X86_PGTABLE_3LEVEL_H +#include + /* * Intel Physical Address Extension (PAE) Mode - three-level page * tables on PPro+ CPUs. @@ -150,10 +152,7 @@ static inline pte_t native_ptep_get_and_clear(pte_t *ptep) { 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; } -- cgit v1.2.3-59-g8ed1b