linux-kernel - Re: [PATCH v3 1/2] KVM: SVM: Increase X2AVIC limit to 4096 vcpus

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Message-ID: <aH6-f1xOMU7dQLsX@google.com>
Date: Mon, 21 Jul 2025 15:26:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Naveen N Rao <naveen@...nel.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Suravee Suthikulpanit <suravee.suthikulpanit@....com>, 
	Vasant Hegde <vasant.hegde@....com>
Subject: Re: [PATCH v3 1/2] KVM: SVM: Increase X2AVIC limit to 4096 vcpus

On Mon, Jul 21, 2025, Naveen N Rao wrote:
> On Fri, Jul 18, 2025 at 07:24:15AM -0700, Sean Christopherson wrote:
> > If there wasn't already an "x2AVIC enabled" print, I would probably lean toward
> > doing nothing.  But since pr_info("x2AVIC enabled\n") already exists, and has
> > plently of free space for adding extra information, there's basically zero downside
> > to printing out the number of supported CPUs.  And it's not just a binary yes/no,
> > e.g. I would wager most people couldn't state the number of vCPUs supported by
> > the "old" x2AVIC.
> 
> Ok, this is what I have now. Let me know if you prefer different 
> wording:
> 
> 	/* AVIC is a prerequisite for x2AVIC. */
>         x2avic_enabled = boot_cpu_has(X86_FEATURE_X2AVIC);
> -       if (x2avic_enabled)
> -               pr_info("x2AVIC enabled\n");
> +       if (x2avic_enabled) {
> +               if (cpu_feature_enabled(X86_FEATURE_X2AVIC_EXT))
> +                       x2avic_max_physical_id = X2AVIC_4K_MAX_PHYSICAL_ID;

I actually like the approach you initially posted, where KVM explicitly sets
x2avic_max_physical_id for both paths.  KVM could obviously rely on global
initialization to set X2AVIC_MAX_PHYSICAL_ID, but it's not like this code is
performance critical, and have all paths in one place makes it easy to understand
what the "default" value is.

		if (cpu_feature_enabled(X86_FEATURE_X2AVIC_EXT))
			x2avic_max_physical_id = X2AVIC_MAX_PHYSICAL_ID_4K;
		else
			x2avic_max_physical_id = X2AVIC_MAX_PHYSICAL_ID;

> +               pr_info("x2AVIC enabled (upto %lld vCPUs)\n", x2avic_max_physical_id + 1);

Maybe s/upto/max?

> +       }
> 
> 
> - Naveen