linux-kernel - Re: [PATCH v2 03/15] KVM: x86: Refresh CPUID on writes to MSR_IA32

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Date:   Mon, 30 Jan 2023 14:33:28 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <jmattson@...gle.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <like.xu.linux@...il.com>, <kan.liang@...ux.intel.com>,
        <wei.w.wang@...el.com>
Subject: Re: [PATCH v2 03/15] KVM: x86: Refresh CPUID on writes to
 MSR_IA32_XSS


On 1/27/2023 3:50 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> Updated CPUID.0xD.0x1, which reports the current required storage size
>> of all features enabled via XCR0 | XSS, when the guest's XSS is modified.
>>
>> Note, KVM does not yet support any XSS based features, i.e. supported_xss
>> is guaranteed to be zero at this time.
>>
>> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
>> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 16 +++++++++++++---
>>   arch/x86/kvm/x86.c   |  6 ++++--
>>   2 files changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6b5912578edd..85e3df6217af 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -272,9 +272,19 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>>   		best->ebx = xstate_required_size(vcpu->arch.xcr0, false);
>>   
>>   	best = cpuid_entry2_find(entries, nent, 0xD, 1);
>> -	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>> -		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
>> -		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
>> +	if (best) {
>> +		if (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>> +		    cpuid_entry_has(best, X86_FEATURE_XSAVEC))  {
>> +			u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
>> +
>> +			best->ebx = xstate_required_size(xstate, true);
>> +		}
>> +
>> +		if (!cpuid_entry_has(best, X86_FEATURE_XSAVES)) {
>> +			best->ecx = 0;
>> +			best->edx = 0;
> ECX and EDX should be left alone, it is userspace's responsibility to provide a
> sane CPUID model.  E.g. KVM doesn't clear EBX or EDX in CPUID.0xD.0x1 when XSAVE
> is unsupported.


Thanks for review!

Sure, I'll remove it.



>
>> +		}
>> +	}
>>   
>>   	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
>>   	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 16726b44061b..888a153e32bc 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3685,8 +3685,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   		 */
>>   		if (data & ~kvm_caps.supported_xss)
>>   			return 1;
>> -		vcpu->arch.ia32_xss = data;
>> -		kvm_update_cpuid_runtime(vcpu);
>> +		if (vcpu->arch.ia32_xss != data) {
>> +			vcpu->arch.ia32_xss = data;
>> +			kvm_update_cpuid_runtime(vcpu);
>> +		}
>>   		break;
>>   	case MSR_SMI_COUNT:
>>   		if (!msr_info->host_initiated)
>> -- 
>> 2.27.0
>>