Re: [PATCH v2 3/4] x86/bugs: KVM: Add support for SRSO_MSR_FIX

Jim Mattson
2025-01-08T19:43:26Z

On Wed, Jan 8, 2025 at 11:15 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Wed, Jan 08, 2025 at 10:37:57AM -0800, Jim Mattson wrote:
> > Surely, IBPB-on-VMexit is worse for performance than safe-RET?!?
>
> We don't need safe-RET with SRSO_USER_KERNEL_NO=1. And there's no safe-RET for
> virt only. So IBPB-on-VMEXIT is the next best thing. The good thing is, those
> machines have BpSpecReduce too so you won't be doing IBPB-on-VMEXIT either but
> what we're talking about here - BpSpecReduce.

I'm suggesting that IBPB-on-VMexit is probably the *worst* thing. If
it weren't for BpSpecReduce, I would want safe-RET for virt only.
(Well, if it weren't for ASI, I would want that, anyway.)

> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette