Re: [PATCH v2 2/7] KVM: X86: Go on updating other CPUID leaves when leaf 1 is absent

[Xiaoyao Li]

On 7/3/2020 3:02 AM, Sean Christopherson wrote:
> On Thu, Jul 02, 2020 at 11:54:03AM -0700, Sean Christopherson wrote:
> > On Tue, Jun 23, 2020 at 07:58:11PM +0800, Xiaoyao Li wrote:
> > > As handling of bits other leaf 1 added over time, kvm_update_cpuid()
> > > should not return directly if leaf 1 is absent, but should go on
> > > updateing other CPUID leaves.
> > >
> > > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> >
> > This should probably be marked for stable.
> >
> > > ---
> > >   arch/x86/kvm/cpuid.c | 23 +++++++++++------------
> > >   1 file changed, 11 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 1d13bad42bf9..0164dac95ef5 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -60,22 +60,21 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> > >   	struct kvm_lapic *apic = vcpu->arch.apic;
> > >   
> > >   	best = kvm_find_cpuid_entry(vcpu, 1, 0);
> > > -	if (!best)
> > > -		return 0;
> >
> > Rather than wrap the existing code, what about throwing it in a separate
> > helper?  That generates an easier to read diff and also has the nice
> > property of getting 'apic' out of the common code.
>
> Hrm, that'd be overkill once the apic code is moved in a few patches.
> What if you keep the cpuid updates wrapped (as in this patch), but then
> do
>
> 	if (best && apic) {
> 	}
>
> for the apic path?  That'll minimize churn for code that is disappearing,
> e.g. will make future git archaeologists happy :-).

Sure. I'll do it in next version.

> > > -
> > > -	/* Update OSXSAVE bit */
> > > -	if (boot_cpu_has(X86_FEATURE_XSAVE) && best->function == 0x1)
> > > -		cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> > > +	if (best) {
> > > +		/* Update OSXSAVE bit */
> > > +		if (boot_cpu_has(X86_FEATURE_XSAVE))
> > > +			cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> > >   				   kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE));
> > >   
> > > -	cpuid_entry_change(best, X86_FEATURE_APIC,
> > > +		cpuid_entry_change(best, X86_FEATURE_APIC,
> > >   			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> > >   
> > > -	if (apic) {
> > > -		if (cpuid_entry_has(best, X86_FEATURE_TSC_DEADLINE_TIMER))
> > > -			apic->lapic_timer.timer_mode_mask = 3 << 17;
> > > -		else
> > > -			apic->lapic_timer.timer_mode_mask = 1 << 17;
> > > +		if (apic) {
> > > +			if (cpuid_entry_has(best, X86_FEATURE_TSC_DEADLINE_TIMER))
> > > +				apic->lapic_timer.timer_mode_mask = 3 << 17;
> > > +			else
> > > +				apic->lapic_timer.timer_mode_mask = 1 << 17;
> > > +		}
> > >   	}
> > >   
> > >   	best = kvm_find_cpuid_entry(vcpu, 7, 0);
> > > --
> > > 2.18.2