On Thu, Aug 20, 2020 at 07:50:50PM -0700, Sean Christopherson wrote: > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index 70dea93378162..fd915c46297c5 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -842,8 +842,13 @@ SYM_CODE_START_LOCAL(paranoid_entry) > * > * The MSR write ensures that no subsequent load is based on a > * mispredicted GSBASE. No extra FENCE required. > + * > + * Disallow RDPID if KVM is enabled as it may consume a guest's TSC_AUX > + * if an NMI arrives in KVM's run loop. KVM loads guest's TSC_AUX on > + * VM-Enter and may not restore the host's value until the CPU returns > + * to userspace, i.e. KVM depends on the kernel not using TSC_AUX. > */ > - SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx > + SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx > no_rdpid=IS_ENABLED(CONFIG_KVM) > ret
With distro configs that's going to be a guaranteed no_rdpid. Also with a grand total of 0 performance numbers that RDPID is even worth it, I'd suggest to just unconditionally remove that thing. Simpler code all-around.
