linux-kernel - Re: [PATCH v2 3/4] x86/bugs: KVM: Add support for SRSO_MSR

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Message-ID: <CALMp9eS1H3pYOmQSE9qPFF2Pk2uvN_hUde=+5sZikBGjAjb+aw@mail.gmail.com>
Date: Wed, 8 Jan 2025 11:43:14 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Sean Christopherson <seanjc@...gle.com>, Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Josh Poimboeuf <jpoimboe@...hat.com>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, KVM <kvm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] x86/bugs: KVM: Add support for SRSO_MSR_FIX

On Wed, Jan 8, 2025 at 11:15 AM Borislav Petkov <bp@...en8.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