linux-kernel - Re: [RFC PATCH v3 1/2] x86: cpu/bugs: add AMD ERAPS support; hardware flushes RSB

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Message-ID: <Z03-FSa6nvVbFjwc@google.com>
Date: Mon, 2 Dec 2024 10:36:05 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Amit Shah <amit@...nel.org>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	x86@...nel.org, linux-doc@...r.kernel.org, thomas.lendacky@....com, 
	bp@...en8.de, tglx@...utronix.de, peterz@...radead.org, jpoimboe@...nel.org, 
	pawan.kumar.gupta@...ux.intel.com, corbet@....net, mingo@...hat.com, 
	dave.hansen@...ux.intel.com, hpa@...or.com, pbonzini@...hat.com, 
	daniel.sneddon@...ux.intel.com, kai.huang@...el.com, sandipan.das@....com, 
	boris.ostrovsky@...cle.com, Babu.Moger@....com, david.kaplan@....com, 
	dwmw@...zon.co.uk, andrew.cooper3@...rix.com, Amit Shah <Amit.Shah@....com>
Subject: Re: [RFC PATCH v3 1/2] x86: cpu/bugs: add AMD ERAPS support; hardware
 flushes RSB

On Mon, Dec 02, 2024, Dave Hansen wrote:
> On 12/2/24 10:09, Amit Shah wrote:
> > I can still include the summary of the discussion in this patch - I
> > just feel it isn't necessary with the rework.
> 
> It's necessary.
> 
> There's a new hardware feature. You want it to replace a software
> sequence in certain situations. You have to make an actual, coherent
> argument argument as to why the hardware feature is a suitable replacement.
> 
> For instance (and I'm pretty sure we've gone over this more than once),
> the changelog here still makes the claim that a "context switch" and a
> "mov-to-CR3" are the same thing.

+1000.  I want a crisp, precise description of the actual hardware behavior, so
that KVM can do the right thing when virtualizing ERAPS.