Re: [PATCH v6 1/2] KVM: x86/cpuid: generalize kvm_update_kvm_cpuid_base() and also capture limit

[Paul Durrant]

On 04/01/2023 19:34, Sean Christopherson wrote:
> On Tue, Dec 20, 2022, Paul Durrant wrote:
> > A sunsequent patch will need to acquire the CPUID leaf range for emulated
> > Xen so explicitly pass the signature of the hypervisor we're interested in
> > to the new function. Also introduce a new kvm_hypervisor_cpuid structure
> > so we can neatly store both the base and limit leaf indices.
> >
> > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > ---
> > Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> >
> > v6:
> >   - New in this version
> > ---
> >   arch/x86/include/asm/kvm_host.h |  7 ++++++-
> >   arch/x86/kvm/cpuid.c            | 15 ++++++++-------
> >   2 files changed, 14 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f35f1ff4427b..ff201ad35551 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -710,6 +710,11 @@ struct kvm_queued_exception {
> >   	bool has_payload;
> >   };
> >   
> > +struct kvm_hypervisor_cpuid {
> > +	u32 base;
> > +	u32 limit;
> > +};
>
> Probably makes sense to place this above "struct kvm_vcpu_xen" right away to
> avoid the (very minor) churn.

Sure.

> >   struct kvm_vcpu_arch {
> >   	/*
> >   	 * rip and regs accesses must go through
> > @@ -826,7 +831,7 @@ struct kvm_vcpu_arch {
> >   
> >   	int cpuid_nent;
> >   	struct kvm_cpuid_entry2 *cpuid_entries;
> > -	u32 kvm_cpuid_base;
> > +	struct kvm_hypervisor_cpuid kvm_cpuid;
> >   
> >   	u64 reserved_gpa_bits;
> >   	int maxphyaddr;
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 0b5bf013fcb8..2468720f8d84 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -180,12 +180,13 @@ static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2
> >   	return 0;
> >   }
> >   
> > -static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
> > +static void kvm_update_hypervisor_cpuid(struct kvm_vcpu *vcpu, const char *hypervisor_signature,
>
> Please wrap.  The 80 char limit is a soft limit, but should still be honored unless
> there's a good reason to run over.

Ok.

> I also vote to name the param "sig" to keep line lengths short.
>
> > +					struct kvm_hypervisor_cpuid *hypervisor_cpuid)
>
> Since the struct is a 64-bit value, what about making this a pure getter that
> returns a copy?
>
> static struct kvm_hypervisor_cpuid kvm_get_hypervisor_cpuid(struct kvm_vcpu *vcpu,
> 							    const char *sig)
> {
> 	struct kvm_hypervisor_cpuid cpuid = {};
> 	struct kvm_cpuid_entry2 *entry;
> 	u32 function;
>
> 	for_each_possible_hypervisor_cpuid_base(cpuid.base) {
> 		entry = kvm_find_cpuid_entry(vcpu, function);
>
> 		if (entry) {
> 			u32 signature[3];
>
> 			signature[0] = entry->ebx;
> 			signature[1] = entry->ecx;
> 			signature[2] = entry->edx;
>
> 			if (!memcmp(signature, sig, sizeof(signature))) {
> 				cpuid.base = function;
> 				cpuid.limit = entry->eax;
> 				break;
> 			}
> 		}
> 	}
>
> 	return cpuid;
> }
>
>
> 	vcpu->arch.kvm_cpuid = kvm_get_hypervisor_cpuid(vcpu, KVM_SIGNATURE);
> 	vcpu->arch.xen.cpuid = kvm_get_hypervisor_cpuid(vcpu, XEN_SIGNATURE);

Yes, if that's preferable then no problem.

  Paul