linux-kernel - Re: [PATCH v6 1/1] x86: kvm: svm: set up ERAPS support for guests

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Message-ID: <aSB-LUfcyx8B9MLr@google.com>
Date: Fri, 21 Nov 2025 06:58:53 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Amit Shah <amit@...nel.org>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	x86@...nel.org, linux-doc@...r.kernel.org, amit.shah@....com, 
	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
Subject: Re: [PATCH v6 1/1] x86: kvm: svm: set up ERAPS support for guests

On Fri, Nov 21, 2025, Andrew Cooper wrote:
> On 20/11/2025 8:11 pm, Sean Christopherson wrote:
> > The emulation requirements are not limited to shadow paging.  From the APM:
> >
> >   The ERAPS feature eliminates the need to execute CALL instructions to clear
> >   the return address predictor in most cases. On processors that support ERAPS,
> >   return addresses from CALL instructions executed in host mode are not used in
> >   guest mode, and vice versa. Additionally, the return address predictor is
> >   cleared in all cases when the TLB is implicitly invalidated (see Section 5.5.3 “TLB
> >   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >   Management,” on page 159) and in the following cases:
> >
> >   • MOV CR3 instruction
> >   • INVPCID other than single address invalidation (operation type 0)
> 
> I already asked AMD for clarification here.  AIUI, INVLPGB should be
> included in this list, and that begs the question what else is missed
> from the documentation.
> 
> >
> > Yes, KVM only intercepts MOV CR3 and INVPCID when NPT is disabled (or INVPCID is
> > unsupported per guest CPUID), but that is an implementation detail, the instructions
> > are still reachable via emulator, and KVM needs to emulate implicit TLB flush
> > behavior.
> 
> The Implicit flushes cover CR0.PG, CR4.{PSE,PGE,PCIDE,PKE}, SMI, RSM,
> writes to MTRR MSR, #INIT, A20M, and "other model specific MSRs, see NDA
> docs".
> 
> The final part is very unhelpful in practice, and necessitates a RAS
> flush on any emulated WRMSR, unless AMD are going to start handing out
> the multi-coloured documents...

Does Xen actually emulate guest TLB flushes on all emulated WRMSRs?  A RAS flush
seems like small peanuts compared to a TLB flush.

> The really fastpath MSRs are unintercepted and won't suffer this overhead.

Heh, if an unintercepted MSR is on the "naughty list", wouldn't that break shadow
paging schemes that rely on intercepting architectural TLB flushes to synchronize
shadow PTEs?