[PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys as allowed in .noinstr

[Sean Christopherson]

On Fri, Oct 10, 2025, Valentin Schneider wrote:
> Later commits will cause objtool to warn about static keys being used in
> .noinstr sections in order to safely defer instruction patching IPIs
> targeted at NOHZ_FULL CPUs.
> 
> These keys are used in .noinstr code, and can be modified at runtime
> (/proc/kernel/vmx* write). However it is not expected that they will be
> flipped during latency-sensitive operations, and thus shouldn't be a source
> of interference wrt the text patching IPI.
>
> Mark it to let objtool know not to warn about it.

Can you elaborate in the changelog on what will happen if the key is toggle?
IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being
patched.  Even just a small footnote like this:

  Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
  being patched, i.e. this is purely about silencing objtool warnings.

to make it clear that there's no bug/race being introduced.

> Reported-by: Josh Poimboeuf <jpoimboe at kernel.org>
> Signed-off-by: Valentin Schneider <vschneid at redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aa157fe5b7b31..dce2bd7375ec8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -204,8 +204,15 @@ module_param(pt_mode, int, S_IRUGO);
>  
>  struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
>  
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
> +/*
> + * Both of these static keys end up being used in .noinstr sections, however
> + * they are only modified:
> + * - at init
> + * - from a /proc/kernel/vmx* write
> + * thus during latency-sensitive operations they should remain stable.
> + */
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_should_flush);
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_flush_cond);
>  static DEFINE_MUTEX(vmx_l1d_flush_mutex);
>  
>  /* Storage for pre module init parameter parsing */
> -- 
> 2.51.0
>