linux-kernel - Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to track "LAM enabled"

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Message-ID: <66235c55-05ac-edd5-c45e-df1c42446eb3@linux.intel.com>
Date:   Wed, 16 Aug 2023 15:08:53 +0800
From:   Binbin Wu <binbin.wu@...ux.intel.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>
Subject: Re: [PATCH v10 3/9] KVM: x86: Use KVM-governed feature framework to
 track "LAM enabled"



On 8/16/2023 11:46 AM, Huang, Kai wrote:
> On Wed, 2023-07-19 at 22:41 +0800, Binbin Wu wrote:
>> Use the governed feature framework to track if Linear Address Masking (LAM)
>> is "enabled", i.e. if LAM can be used by the guest. So that guest_can_use()
>> can be used to support LAM virtualization.
> Better to explain why to use governed feature for LAM?  Is it because there's
> hot path(s) calling guest_cpuid_has()?  Anyway some context of why can help
> here.
Yes, to avoid calling guest_cpuid_has() in CR3 handling and instruction 
emulation paths.
I will add the context next version.
Thanks!

>
>> LAM modifies the checking that is applied to 64-bit linear addresses, allowing
>> software to use of the untranslated address bits for metadata and masks the
>> metadata bits before using them as linear addresses to access memory.
>>
>> Signed-off-by: Binbin Wu <binbin.wu@...ux.intel.com>
>> ---
>>   arch/x86/kvm/governed_features.h | 2 ++
>>   arch/x86/kvm/vmx/vmx.c           | 3 +++
>>   2 files changed, 5 insertions(+)
>>
>> diff --git a/arch/x86/kvm/governed_features.h b/arch/x86/kvm/governed_features.h
>> index 40ce8e6608cd..708578d60e6f 100644
>> --- a/arch/x86/kvm/governed_features.h
>> +++ b/arch/x86/kvm/governed_features.h
>> @@ -5,5 +5,7 @@ BUILD_BUG()
>>   
>>   #define KVM_GOVERNED_X86_FEATURE(x) KVM_GOVERNED_FEATURE(X86_FEATURE_##x)
>>   
>> +KVM_GOVERNED_X86_FEATURE(LAM)
>> +
>>   #undef KVM_GOVERNED_X86_FEATURE
>>   #undef KVM_GOVERNED_FEATURE
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 0ecf4be2c6af..ae47303c88d7 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7783,6 +7783,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>>   		vmx->msr_ia32_feature_control_valid_bits &=
>>   			~FEAT_CTL_SGX_LC_ENABLED;
>>   
>> +	if (boot_cpu_has(X86_FEATURE_LAM))
>> +		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
>> +
> If you want to use boot_cpu_has(), it's better to be done at your last patch to
> only set the cap bit when boot_cpu_has() is true, I suppose.
Yes, but new version of kvm_governed_feature_check_and_set() of 
KVM-governed feature framework will check against kvm_cpu_cap_has() as well.
I will remove the if statement and call 
kvm_governed_feature_check_and_set()  directly.
https://lore.kernel.org/kvm/20230815203653.519297-2-seanjc@google.com/


>
>>   	/* Refresh #PF interception to account for MAXPHYADDR changes. */
>>   	vmx_update_exception_bitmap(vcpu);
>>   }