linux-kernel - Re: [PATCH 3/9] KVM: x86: Initialize guest cpu

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Message-ID: <20231121031024.gxghavohrkgh7psd@yy-desk-7060>
Date:   Tue, 21 Nov 2023 11:10:24 +0800
From:   Yuan Yao <yuan.yao@...ux.intel.com>
To:     "Yang, Weijiang" <weijiang.yang@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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 3/9] KVM: x86: Initialize guest cpu_caps based on guest
 CPUID

On Fri, Nov 17, 2023 at 04:33:27PM +0800, Yang, Weijiang wrote:
> On 11/17/2023 6:29 AM, Sean Christopherson wrote:
> > On Thu, Nov 16, 2023, Weijiang Yang wrote:
> > > On 11/11/2023 7:55 AM, Sean Christopherson wrote:
> > >
> > > [...]
> > >
> > > > -static __always_inline void guest_cpu_cap_check_and_set(struct kvm_vcpu *vcpu,
> > > > -							unsigned int x86_feature)
> > > > +static __always_inline void guest_cpu_cap_clear(struct kvm_vcpu *vcpu,
> > > > +						unsigned int x86_feature)
> > > >    {
> > > > -	if (kvm_cpu_cap_has(x86_feature) && guest_cpuid_has(vcpu, x86_feature))
> > > > +	unsigned int x86_leaf = __feature_leaf(x86_feature);
> > > > +
> > > > +	reverse_cpuid_check(x86_leaf);
> > > > +	vcpu->arch.cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
> > > > +}
> > > > +
> > > > +static __always_inline void guest_cpu_cap_change(struct kvm_vcpu *vcpu,
> > > > +						 unsigned int x86_feature,
> > > > +						 bool guest_has_cap)
> > > > +{
> > > > +	if (guest_has_cap)
> > > >    		guest_cpu_cap_set(vcpu, x86_feature);
> > > > +	else
> > > > +		guest_cpu_cap_clear(vcpu, x86_feature);
> > > > +}
> > > I don't see any necessity to add 3 functions, i.e., guest_cpu_cap_{set, clear, change}, for
> > I want to have equivalents to the cpuid_entry_*() APIs so that we don't end up
> > with two different sets of names.  And the clear() API already has a second user.
> >
> > > guest_cpu_cap update. IMHO one function is enough, e.g,:
> > Hrm, I open coded the OR/AND logic in cpuid_entry_change() to try to force CMOV
> > instead of Jcc.  That honestly seems like a pointless optimization.  I would
> > rather use the helpers, which is less code.
> >
> > > static __always_inline void guest_cpu_cap_update(struct kvm_vcpu *vcpu,
> > >                                                   unsigned int x86_feature,
> > >                                                   bool guest_has_cap)
> > > {
> > >          unsigned int x86_leaf = __feature_leaf(x86_feature);
> > >
> > > reverse_cpuid_check(x86_leaf);
> > >          if (guest_has_cap)
> > >                  vcpu->arch.cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
> > > else
> > >                  vcpu->arch.cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
> > > }
> > >
> > > > +
> > > > +static __always_inline void guest_cpu_cap_restrict(struct kvm_vcpu *vcpu,
> > > > +						   unsigned int x86_feature)
> > > > +{
> > > > +	if (!kvm_cpu_cap_has(x86_feature))
> > > > +		guest_cpu_cap_clear(vcpu, x86_feature);
> > > >    }
> > > _restrict is not clear to me for what the function actually does -- it
> > > conditionally clears guest cap depending on KVM support of the feature.
> > >
> > > How about renaming it to guest_cpu_cap_sync()?
> > "sync" isn't correct because it's not synchronizing with KVM's capabilitiy, e.g.
> > the guest capability will remaing unset if the guest CPUID bit is clear but the
> > KVM capability is available.
> >
> > How about constrain()?
> I don't know, just feel we already have guest_cpu_cap_{set, clear, change}, here the name cannot exactly match the behavior of the function, maybe guest_cpu_cap_filter()? But just ignore the nit, up to you to decide the name :-)

How about guest_cpu_cap_kvm_restrict or guest_cpu_cap_kvm_constrain ?

>
>