linux-kernel - Re: [PATCH v6 1/2] KVM: x86/cpuid: generalize kvm_update_kvm_cpuid

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Date:   Thu, 5 Jan 2023 10:38:08 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paul Durrant <pdurrant@...zon.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v6 1/2] KVM: x86/cpuid: generalize
 kvm_update_kvm_cpuid_base() and also capture limit

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@...zon.com>
>> ---
>> Cc: Sean Christopherson <seanjc@...gle.com>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Borislav Petkov <bp@...en8.de>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: David Woodhouse <dwmw2@...radead.org>
>>
>> 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