linux-kernel - Re: [PATCH v2 11/15] KVM: x86: Add XSAVE Support for Architectural LBR

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Date:   Mon, 30 Jan 2023 21:13:11 +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>, Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v2 11/15] KVM: x86: Add XSAVE Support for Architectural
 LBR


On 1/28/2023 6:07 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> From: Like Xu <like.xu@...ux.intel.com>
>>
>> On processors supporting XSAVES and XRSTORS, Architectural LBR XSAVE
>> support is enumerated from CPUID.(EAX=0DH, ECX=1):ECX[bit 15].
>> The detailed sub-leaf for Arch LBR is enumerated in CPUID.(0DH, 0FH).
>>
>> XSAVES provides a faster means than RDMSR for guest to read all LBRs.
>> When guest IA32_XSS[bit 15] is set, the Arch LBR state can be saved using
>> XSAVES and restored by XRSTORS with the appropriate RFBM.
>>
>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 4 ++++
>>   arch/x86/kvm/x86.c     | 2 +-
>>   2 files changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 359da38a19a1..3bc892e8cf7a 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7733,6 +7733,10 @@ static __init void vmx_set_cpu_caps(void)
>>   		kvm_cpu_cap_check_and_set(X86_FEATURE_DS);
>>   		kvm_cpu_cap_check_and_set(X86_FEATURE_DTES64);
>>   	}
>> +	if (!cpu_has_vmx_arch_lbr()) {
>> +		kvm_cpu_cap_clear(X86_FEATURE_ARCH_LBR);
> No, this needs to be opt-in, not opt-out.  I.e. omit the flag from common CPUID
> code and set it if and only if it's fully supported.  It's not out of the realm
> of possibilities that AMD might want to support arch LBRs, at which point those
> CPUs would explode.

Will modify this patch.

>
>> +		kvm_caps.supported_xss &= ~XFEATURE_MASK_LBR;
>> +	}
>>   
>>   	if (!enable_pmu)
>>   		kvm_cpu_cap_clear(X86_FEATURE_PDCM);
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 889be0c9176d..38df08d9d0cb 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -217,7 +217,7 @@ 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
>> +#define KVM_SUPPORTED_XSS     XFEATURE_MASK_LBR
>>   
>>   u64 __read_mostly host_efer;
>>   EXPORT_SYMBOL_GPL(host_efer);
>> -- 
>> 2.27.0
>>