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 9:47 PM UTC, Sean Christopherson wrote:
> On Fri, Oct 31, 2025, Brendan Jackman wrote:
>> On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
>> > 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.
>
> No objection from me if we want strong guarantees that CLEAR_CPU_BUF_VM and
> CLEAR_CPU_BUF_MMIO are mutually exclusive.  Though I do think the KVM side of
> things (and the kernel in general) should be paranoid and not lean _too_ hard
> on such assumptions.

Ah, after finishing the review I realised these are _not_ actually
mutually exclusive in terms of the implementation. So asserting here
that they are mutually exclusive would just be confusing, rather than
helfpul, IMO.