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

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Message-ID: <20250217160728.GFZ7NewJHpMaWdiX2M@fat_crate.local>
Date: Mon, 17 Feb 2025 17:07:28 +0100
From: Borislav Petkov <bp@...en8.de>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Patrick Bellasi <derkling@...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>
Subject: Re: Re: Re: Re: [PATCH] x86/bugs: KVM: Add support for SRSO_MSR_FIX

On Sun, Feb 16, 2025 at 09:59:59PM -0800, Yosry Ahmed wrote:
> If 1-2% is the cost for keeping the MSR enabled at all times, I wonder
> if we should just do that for simplicitly, and have it its own
> mitigation option (chosen by the cmdline).

Yes, that's what David and I think we should do initially.

Then we can chase some more involved scheme like setting the bit before the
first VM runs and then clearing it when the last one exits. I *think* I've
seen something like that in KVM but I need to stare in detail.

> - Upon return to userspace (similar to your previous proposal). In this
>   case we run userspace with the MSR cleared, and only perform an IBPB
>   in the exit to userspace pace.

You want to IBPB before clearing the MSR as otherwise host kernel will be
running with the mistrained gunk from the guest.

> Any thoughts? 

Yes, let's keep it simple and do anything more involved *only* if it is really
necessary.

-- 
Regards/Gruss,
    Boris.

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