[PATCH v1 0/3] x86: Write FRED RSP0 on return to userspace

["Xin Li (Intel)" <xin@zytor.com>]

This patch set moves writing MSR_IA32_FRED_RSP0 to return to userspace
from context switch.

In the discussion of save/restore host/guest FRED RSP0 for KVM, Sean
proposed to have the kernel write MSR_IA32_FRED_RSP0 on return to
userspace, i.e., arch_exit_to_user_mode_prepare(), instead of on context
switch. [1]

hpa suggested to test ti_work for zero and then process individual bits
in arch_exit_to_user_mode_prepare.  And a quick measurement shows that
in most cases, ti_work values passed to arch_exit_to_user_mode_prepare()
are zeros, e.g., 99% in kernel build tests.  This zero test change was
then sent to Intel 0day tests, and no perf regression is reported.

Besides, per the discussion of how to write MSR_IA32_FRED_RSP0 with the
introduction of WRMSRNS [2], use the alternatives mechanism to choose
WRMSRNS when it's available, otherwise fallback to WRMSR.

[1] https://lore.kernel.org/lkml/ZpkfkSMPiXrS9r2K@google.com/
[2] https://lore.kernel.org/lkml/15f56e6a-6edd-43d0-8e83-bb6430096514@citrix.com/


Andrew Cooper (1):
  x86/msr: Switch between WRMSRNS and WRMSR with the alternatives
    mechanism

Xin Li (Intel) (2):
  x86/entry: Test ti_work for zero before processing individual bits
  x86/entry: Set FRED RSP0 on return to userspace instead of context
    switch

 arch/x86/include/asm/entry-common.h | 21 ++++++++++++++-------
 arch/x86/include/asm/msr.h          | 28 ++++++++++++++--------------
 arch/x86/include/asm/switch_to.h    |  3 +--
 arch/x86/include/asm/thread_info.h  |  2 ++
 arch/x86/kernel/cpu/cpuid-deps.c    |  1 -
 5 files changed, 31 insertions(+), 24 deletions(-)


base-commit: 9ebdc7589cbb5c976e6c8807cbe13f263d70d32c
-- 
2.45.2