linux-kernel - Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write

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Message-ID: <fqkt676ogwaagsdcscpdw3p5i3nkp2ka5vf4hlkxtd6qq7j35y@vsnt3nrgmmo5>
Date: Wed, 2 Apr 2025 19:17:59 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jim Mattson <jmattson@...gle.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, amit@...nel.org, 
	kvm@...r.kernel.org, amit.shah@....com, thomas.lendacky@....com, bp@...en8.de, 
	tglx@...utronix.de, peterz@...radead.org, pawan.kumar.gupta@...ux.intel.com, 
	corbet@....net, mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com, 
	seanjc@...gle.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
Subject: Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write_ibpb() if applicable

On Wed, Apr 02, 2025 at 02:04:04PM -0700, Jim Mattson wrote:
> On Wed, Apr 2, 2025 at 11:20 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >
> > __write_ibpb() does IBPB, which (among other things) flushes branch type
> > predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
> > has been disabled, branch type flushing isn't needed, in which case the
> > lighter-weight SBPB can be used.
> 
> When nested SVM is not supported, should KVM "promote"
> SRSO_USER_KERNEL_NO on the host to SRSO_NO in KVM_GET_SUPPORTED_CPUID?
> Or is a Linux guest clever enough to do the promotion itself if
> CPUID.80000001H:ECX.SVM[bit 2] is clear?

I'm afraid that question is beyond my pay grade, maybe some AMD or virt
folks can chime in.

-- 
Josh