Re: [PATCH 1/3] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

[Pawan Gupta]

On Thu, Oct 30, 2025 at 12:28:06PM +0000, Brendan Jackman wrote:
> On Wed Oct 29, 2025 at 9:26 PM UTC, Pawan Gupta wrote:
> > TSA mitigation:
> >
> >   d8010d4ba43e ("x86/bugs: Add a Transient Scheduler Attacks mitigation")
> >
> > introduced VM_CLEAR_CPU_BUFFERS for guests on AMD CPUs. Currently on Intel
> > CLEAR_CPU_BUFFERS is being used for guests which has a much broader scope
> > (kernel->user also).
> >
> > Make mitigations on Intel consistent with TSA. This would help handling the
> > guest-only mitigations better in future.
> >
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 9 +++++++--
> >  arch/x86/kvm/vmx/vmenter.S | 3 ++-
> >  2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index d7fa03bf51b4517c12cc68e7c441f7589a4983d1..6d00a9ea7b4f28da291114a7a096b26cc129b57e 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -194,7 +194,7 @@ 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, and should only be enabled when KVM-only
> > + * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
> >   * mitigation is required.
> >   */
> 
> So if I understand correctly with this patch the aim is:
> 
> X86_FEATURE_CLEAR_CPU_BUF means verw before exit to usermode
> 
> X86_FEATURE_CLEAR_CPU_BUF_VM means unconditional verw before VM Enter
> 
> cpu_buf_vm_clear[_mmio_only] means verw before VM Enter for
> MMIO-capable guests.

Yup, thats the goal.

> Since this is being cleaned up can we also:
> 
> - Update the definition of X86_FEATURE_CLEAR_CPU_BUF in cpufeatures.h to
>   say what context it applies to (now it's specifically exit to user)
> 
> - Clear up how verw_clear_cpu_buf_mitigation_selected relates to these
>   two flags. Thinking aloud here... it looks like this is set:
> 
>   - If MDS mitigations are on, meaning both flags are set
> 
>   - If TAA mitigations are on, meaning both flags are set
> 
>   - If MMIO mitigations are on, and the CPU has MDS or TAA. In this case
>     both flags are set, but this causality is messier.
> 
>   - If RFDS mitigations are on and supported, meaning both flags are set
> 
>   So if I'm reading this correctly whenever
>   verw_clear_cpu_buf_mitigation_selected we should expect both flags
>   enabled. So I think all that's needed is to add a reference to
>   X86_FEATURE_CLEAR_CPU_BUF_VM to the comment?

Yes. I will update the comment accordingly.

> I think we also need to update the assertion of vmx->disable_fb_clear?

I am not quite sure about the update needed. Could you please clarify?

> Anyway thanks this seems like a very clear improvement to me.

Thanks for the review and suggestions!