linux-kernel - Re: [RFC PATCH] KVM: x86: Advertise PCID based on hardware support (with an asterisk)

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Message-ID: <Zhg18kSHFeWN1xJH@google.com>
Date: Thu, 11 Apr 2024 12:11:46 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, Michael Kelley <mhklinux@...look.com>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Andrew Cooper <andrew.cooper3@...rix.com>, Xi Ruoyao <xry111@...111.site>
Subject: Re: [RFC PATCH] KVM: x86: Advertise PCID based on hardware support
 (with an asterisk)

On Thu, Apr 11, 2024, Paolo Bonzini wrote:
> On 4/11/24 18:31, Sean Christopherson wrote:
> > Force set a synthetic feature, GUEST_PCID, if PCID can be safely used in
> > virtual machines, even if the kernel itself disables PCID support, and
> > advertise PCID support in KVM if GUEST_PCID is set.
> > 
> > When running on a CPU that is affected by Intel's "Global INVLPG" erratum,
> > which does NOT affect VMX non-root mode, it is safe to virtualize PCID for
> > KVM guests, even though it is not safe for the kernel itself to enable PCID.
> > Ditto for if the kernel disables PCID because CR4.PGE isn't supported.
> 
> But the guest would not use it if the f/m/s matches, right?

Maybe?  There's another in-flight patch for dealing with the guest side of
things.

https://lore.kernel.org/all/20240411144322.14585-2-xry111@xry111.site

> If the advantage is basically not splitting the migration pool, is that a
> concern for the affected Alder Lake/Gracemont/Raptor Lake processors?

I have put _zero_ thought into what value this actually adds (another reason I
tagged it RFC).  This was purely a "it's easy, so why not".