linux-kernel - Re: [PATCH 6/9] KVM: x86: Update guest cpu_caps at runtime for dynamic CPUID-based features

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Message-ID: <ZVaVXtKU1nxjFkrw@google.com>
Date:   Thu, 16 Nov 2023 14:19:10 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Weijiang Yang <weijiang.yang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 6/9] KVM: x86: Update guest cpu_caps at runtime for
 dynamic CPUID-based features

On Thu, Nov 16, 2023, Weijiang Yang wrote:
> On 11/11/2023 7:55 AM, Sean Christopherson wrote:
> >   static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
> >   				       int nent)
> >   {
> >   	struct kvm_cpuid_entry2 *best;
> > +	struct kvm_vcpu *caps = vcpu;
> > +
> > +	/*
> > +	 * Don't update vCPU capabilities if KVM is updating CPUID entries that
> > +	 * are coming in from userspace!
> > +	 */
> > +	if (entries != vcpu->arch.cpuid_entries)
> > +		caps = NULL;
> 
> Nit, why here we use caps instead of vcpu? Looks a bit weird.

See my response to Robert: https://lore.kernel.org/all/9395d416-cc5c-536d-641e-ffd971b682d1@gmail.com

> >   	best = cpuid_entry2_find(entries, nent, 1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> > -	if (best) {
> > -		/* Update OSXSAVE bit */
> > -		if (boot_cpu_has(X86_FEATURE_XSAVE))
> > -			cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> > +
> > +	if (boot_cpu_has(X86_FEATURE_XSAVE))
> > +		kvm_update_feature_runtime(caps, best, X86_FEATURE_OSXSAVE,
> >   					   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
> > -		cpuid_entry_change(best, X86_FEATURE_APIC,
> > -			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> > +	kvm_update_feature_runtime(caps, best, X86_FEATURE_APIC,
> > +				   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> > -		if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
> > -			cpuid_entry_change(best, X86_FEATURE_MWAIT,
> > -					   vcpu->arch.ia32_misc_enable_msr &
> > -					   MSR_IA32_MISC_ENABLE_MWAIT);
> > -	}
> > +	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
> > +		kvm_update_feature_runtime(caps, best, X86_FEATURE_MWAIT,
> > +					   vcpu->arch.ia32_misc_enable_msr & MSR_IA32_MISC_ENABLE_MWAIT);
> 
> > 80 characters?

Hmm, yeah, I suppose.