[PATCH v2 0/2] x86/cpu/AMD: Make LFENCE a serializing instruction on AMD

[Tom Lendacky]

To aid in speculation control, the LFENCE instruction will be turned into
a serializing instruction. There is less performance impact using LFENCE
in this way compared to MFENCE.

With LFENCE now being a serializing instruction, it can be also used in
rdtsc_ordered() in preference to MFENCE_RDTSC.  Since the kernel could
be running under a hypervisor that does not allow writing to that MSR,
it must be first verified that the write was successful before setting
the LFENCE_RDTSC feature.

The following patches are included in this series:
- Make LFENCE a serializing instruction on AMD
- Use LFENCE_RDTSC in preference to MFENCE_RDTSC on AMD

This patch series is based on tip:x86/pti.

---

Changes from v1:
- Add a check verifying the MSR was actually updated
- Remove the third patch that eliminates the MFENCE_RDTSC feature
  (since the feature is still needed)
- Adding Dan Williams to the cc since this will impact nospec_barrier(),
  which will require an alternative_2 to add an MFENCE instruction with
  an MFENCE_RDTSC check

Tom Lendacky (2):
      x86/cpu/AMD: Make LFENCE a serializing instruction
      x86/cpu/AMD: Use LFENCE_RDTSC in preference to MFENCE_RDTSC


 arch/x86/include/asm/msr-index.h |    3 +++
 arch/x86/kernel/cpu/amd.c        |   27 +++++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

-- 
Tom Lendacky