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

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Message-ID: <bd8e7f8b-532f-4372-a3fd-69893e359b42@redhat.com>
Date: Thu, 11 Apr 2024 18:36:46 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
 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>
Cc: 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 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?  If the 
advantage is basically not splitting the migration pool, is that a 
concern for the affected Alder Lake/Gracemont/Raptor Lake processors?

Paolo