On Mon, Nov 19, 2018 at 10:46 AM Borislav Petkov <b...@alien8.de> wrote: > > From: Borislav Petkov <b...@suse.de> > > Currently, the kernel uses > > [LM]FENCE; RDTSC > > in the timekeeping code, to guarantee monotonicity of time where the > *FENCE is selected based on vendor. > > Replace that sequence with RDTSCP which is faster or on-par and gives > the same guarantees. > > A microbenchmark on Intel shows that the change is on-par. > > On AMD, the change is either on-par with the current LFENCE-prefixed > RDTSC and some are slightly better with RDTSCP. > > The comparison is done with the LFENCE-prefixed RDTSC (and not with the > MFENCE-prefixed one, as one would normally expect) because all modern > AMD families make LFENCE serializing and thus avoid the heavy MFENCE by > effectively enabling X86_FEATURE_LFENCE_RDTSC. >
I thought I benchmarked this on Intel at some point and found the LFENCE;RDTSC variant to be slightly faster. But I believe you, so: Acked-by: Andy Lutomirski <l...@kernel.org>
