Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write_ibpb() if applicable

[Josh Poimboeuf]

On Wed, Apr 02, 2025 at 02:04:04PM -0700, Jim Mattson wrote:
> On Wed, Apr 2, 2025 at 11:20 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > __write_ibpb() does IBPB, which (among other things) flushes branch type
> > predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
> > has been disabled, branch type flushing isn't needed, in which case the
> > lighter-weight SBPB can be used.
> 
> When nested SVM is not supported, should KVM "promote"
> SRSO_USER_KERNEL_NO on the host to SRSO_NO in KVM_GET_SUPPORTED_CPUID?
> Or is a Linux guest clever enough to do the promotion itself if
> CPUID.80000001H:ECX.SVM[bit 2] is clear?

I'm afraid that question is beyond my pay grade, maybe some AMD or virt
folks can chime in.

-- 
Josh