linux-kernel - Re: [PATCH v5 05/19] KVM:x86: Initialize kvm_caps.supported

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Date:   Tue, 8 Aug 2023 23:08:00 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <chao.gao@...el.com>, <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v5 05/19] KVM:x86: Initialize kvm_caps.supported_xss

On 8/5/2023 2:45 AM, Sean Christopherson wrote:
> On Thu, Aug 03, 2023, Yang Weijiang wrote:
>> Set kvm_caps.supported_xss to host_xss && KVM XSS mask.
>> host_xss contains the host supported xstate feature bits for thread
>> context switch, KVM_SUPPORTED_XSS includes all KVM enabled XSS feature
>> bits, the operation result represents all KVM supported feature bits.
>> Since the result is subset of host_xss, the related XSAVE-managed MSRs
>> are automatically swapped for guest and host when vCPU exits to
>> userspace.
>>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 1 -
>>   arch/x86/kvm/x86.c     | 6 +++++-
>>   2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 0ecf4be2c6af..c8d9870cfecb 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7849,7 +7849,6 @@ static __init void vmx_set_cpu_caps(void)
>>   		kvm_cpu_cap_set(X86_FEATURE_UMIP);
>>   
>>   	/* CPUID 0xD.1 */
>> -	kvm_caps.supported_xss = 0;
> Dropping this code in *this* patch is wrong, this belong in whatever patch(es) adds
> IBT and SHSTK support in VMX.
>
> And that does matter because it means this common patch can be carried wih SVM
> support without breaking VMX.
OK, I'll dropping this line for VMX/SVM in CET feature bits enabling patch.
>>   	if (!cpu_has_vmx_xsaves())
>>   		kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
>>   
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 5d6d6fa33e5b..e9f3627d5fdd 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -225,6 +225,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
>>   				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
>>   				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>>   
>> +#define KVM_SUPPORTED_XSS     0
>> +
>>   u64 __read_mostly host_efer;
>>   EXPORT_SYMBOL_GPL(host_efer);
>>   
>> @@ -9498,8 +9500,10 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>>   
>>   	rdmsrl_safe(MSR_EFER, &host_efer);
>>   
>> -	if (boot_cpu_has(X86_FEATURE_XSAVES))
>> +	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
>>   		rdmsrl(MSR_IA32_XSS, host_xss);
>> +		kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
>> +	}
> Can you opportunistically (in this patch) hoist this above EFER so that XCR0 and
> XSS are colocated?  I.e. end up with this:
>
> 	if (boot_cpu_has(X86_FEATURE_XSAVE)) {
> 		host_xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
> 		kvm_caps.supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
> 	}
> 	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> 		rdmsrl(MSR_IA32_XSS, host_xss);
> 		kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
> 	}
>
> 	rdmsrl_safe(MSR_EFER, &host_efer);
Will change it, thanks!