linux-kernel - Re: [PATCH v4 18/21] kvm: x86: Add support for getting/setting expanded xstate buffer

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Message-ID: <c41a0458-1bfa-9dc6-71ce-f0433cd400e7@redhat.com>
Date:   Tue, 4 Jan 2022 21:45:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yang Zhong <yang.zhong@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, corbet@....net, shuah@...nel.org,
        jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        guang.zeng@...el.com, wei.w.wang@...el.com
Subject: Re: [PATCH v4 18/21] kvm: x86: Add support for getting/setting
 expanded xstate buffer

On 1/4/22 20:46, Sean Christopherson wrote:
> On Wed, Dec 29, 2021, Yang Zhong wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index bdf89c28d2ce..76e1941db223 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4296,6 +4296,11 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>   		else
>>   			r = 0;
>>   		break;
>> +	case KVM_CAP_XSAVE2:
>> +		r = kvm->vcpus[0]->arch.guest_fpu.uabi_size;
> 
> a) This does not compile against kvm/queue.
> 
>     arch/x86/kvm/x86.c: In function ‘kvm_vm_ioctl_check_extension’:
>     arch/x86/kvm/x86.c:4317:24: error: ‘struct kvm’ has no member named ‘vcpus’
>      4317 |                 r = kvm->vcpus[0]->arch.guest_fpu.uabi_size;
> 
> b) vcpu0 is not guaranteed to be non-NULL at this point.

Yang, you can post an incremental patch for this.  You can use the 
highest bit of the guest-permitted xcr0 (i.e. the OR of KVM's supported 
XCR0 an the guest-permitted dynamic features) and pass it to cpuid(0xD).

Paolo