Re: [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO Stale Data mitigation

[Brendan Jackman <jackmanb@google.com>]

On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> Convert the MMIO Stale Data mitigation flag from a static branch into an
> X86_FEATURE_xxx so that it can be used via ALTERNATIVE_2 in KVM.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/include/asm/cpufeatures.h   |  1 +
>  arch/x86/include/asm/nospec-branch.h |  2 --
>  arch/x86/kernel/cpu/bugs.c           | 11 +----------
>  arch/x86/kvm/mmu/spte.c              |  2 +-
>  arch/x86/kvm/vmx/vmx.c               |  4 ++--
>  5 files changed, 5 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 7129eb44adad..d1d7b5ec6425 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -501,6 +501,7 @@
>  #define X86_FEATURE_ABMC		(21*32+15) /* Assignable Bandwidth Monitoring Counters */
>  #define X86_FEATURE_MSR_IMM		(21*32+16) /* MSR immediate form instructions */
>  #define X86_FEATURE_X2AVIC_EXT		(21*32+17) /* AMD SVM x2AVIC support for 4k vCPUs */
> +#define X86_FEATURE_CLEAR_CPU_BUF_MMIO	(21*32+18) /* Clear CPU buffers using VERW before VMRUN, iff the vCPU can access host MMIO*/
>  
>  /*
>   * BUG word(s)
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 923ae21cbef1..b29df45b1edb 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -579,8 +579,6 @@ DECLARE_STATIC_KEY_FALSE(cpu_buf_idle_clear);
>  
>  DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
>  
> -DECLARE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
> -
>  extern u16 x86_verw_sel;
>  
>  #include <asm/segment.h>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 723666a1357e..9acf6343b0ac 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -192,14 +192,6 @@ EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
>   */
>  DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
>  
> -/*
> - * Controls CPU Fill buffer clear before VMenter. This is a subset of
> - * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
> - * mitigation is required.
> - */

This comment wasn't super clear IMO but now that we're losing it, maybe
we can replace it with a WARN_ON() at the end of
cpu_apply_mitigations() or something (maybe it belongs in VMX code)? To
make it more obvious that X86_FEATURE_CLEAR_CPU_BUF_VM and
X86_FEATURE_CLEAR_CPU_BUF_MMIO are mutually exclusive.

Other than the continued bikeshedding,

Reviewed-by: Brendan Jackman <jackmanb@google.com>