mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's 2 to 3 times slower than lock; addl that we use on older CPUs. So we really should use the locked variant everywhere, except that intel manual says that clflush is only ordered by mfence, so we can't. Note: some callers of clflush seems to assume sfence will order it, so there could be existing bugs around this code. Fortunately no callers of clflush (except one) order it using smp_mb(), so after fixing that one caller, it seems safe to override smp_mb straight away. Down the road, it might make sense to introduce clflush_mb() and switch to that for clflush callers. While I was at it, I found some inconsistencies in comments in arch/x86/include/asm/barrier.h The documentation fixes are included first - I verified that they do not change the generated code at all. Borislav Petkov said they will appear in tip eventually, included here for completeness. The last patch changes __smp_mb() to lock addl. I was unable to measure a speed difference on a macro benchmark, but I noted that even doing #define mb() barrier() seems to make no difference for most benchmarks (it causes hangs sometimes, of course). HPA asked that the last patch is deferred until we hear back from intel, which makes sense of course. So it needs HPA's ack. Changes from v3: Leave mb() alone for now since it's used to order clflush, which requires mfence. Optimize smp_mb instead. Changes from v2: add patch adding cc clobber for addl tweak commit log for patch 2 use addl at SP-4 (as opposed to SP) to reduce data dependencies Michael S. Tsirkin (5): x86: add cc clobber for addl x86: drop a comment left over from X86_OOSTORE x86: tweak the comment about use of wmb for IO x86: use mb() around clflush x86: drop mfence in favor of lock+addl arch/x86/include/asm/barrier.h | 17 ++++++++--------- arch/x86/kernel/process.c | 4 ++-- 2 files changed, 10 insertions(+), 11 deletions(-) -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
