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

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Message-ID: <20250214201005.GBZ6-jHUff99tmkyBK@fat_crate.local>
Date: Fri, 14 Feb 2025 21:10:05 +0100
From: Borislav Petkov <bp@...en8.de>
To: Patrick Bellasi <derkling@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Patrick Bellasi <derkling@...bug.net>,
	Brendan Jackman <jackmanb@...gle.com>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: Re: Re: Re: [PATCH] x86/bugs: KVM: Add support for SRSO_MSR_FIX

On Thu, Feb 13, 2025 at 05:50:57PM +0000, Patrick Bellasi wrote:
> The "should be set identically across all processors in the system" makes me
> wondering if using the "KVM's user_return approach" proposed here is robust
> enough. Could this not lead to the bit being possibly set only on some CPU
> but not others?

That's fine, we should update that paper.

> If BpSpecReduce does not prevent training, but only the training from being
> used, should not we keep it consistently set after a guest has run, or until an
> IBPB is executed?

After talking with folks internally, you're probably right. We should slap an
IBPB before clearing. Which means, I cannot use the MSR return slots anymore.
I will have to resurrect some of the other solutions we had lined up...

Stay tuned.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette