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

[Sean Christopherson <seanjc@xxxxxxxxxx>]

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.

>  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.

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);