Re: [PATCH v2 1/3] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps

[Chao Gao]

On Wed, May 24, 2023 at 04:14:10PM +0800, Xiaoyao Li wrote:
>On 5/24/2023 2:16 PM, Chao Gao wrote:
>> to avoid computing the supported value at runtime every time.
>> 
>> Toggle the ARCH_CAP_SKIP_VMENTRY_L1DFLUSH bit when l1tf_vmx_mitigation
>> is modified to achieve the same result as runtime computing.
>
>It's not the same result.

it is because ...

>
>In kvm_get_arch_capabilities(), host's value is honored. I.e., when host
>supports ARCH_CAP_SKIP_VMENTRY_L1DFLUSH, l1tf_vmx_mitigation doesn't make any
>difference to the result.

... l1tf_vmx_mitigation should be VMENTER_L1D_FLUSH_NOT_REQUIRED in this
case. l1tf_vmx_mitigation cannot be VMENTER_L1D_FLUSH_NEVER.

>
>> Opportunistically, add a comment to document the problem of allowing
>> changing the supported value of ARCH_CAPABILITIES and the reason why
>> we don't fix it.
>> 
>> No functional change intended.
>> 
>> Link: https://lore.kernel.org/all/ZGZhW%2Fx5OWPmx1qD@xxxxxxxxxx/
>> Link: https://lore.kernel.org/all/ZGeU9sYTPxqNGSqI@xxxxxxxxxx/
>> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 25 +++++++++++++++++++++++--
>>   arch/x86/kvm/x86.c     |  7 ++++---
>>   arch/x86/kvm/x86.h     |  1 +
>>   3 files changed, 28 insertions(+), 5 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 44fb619803b8..8274ef5e89e5 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -309,10 +309,31 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
>>   	l1tf_vmx_mitigation = l1tf;
>> -	if (l1tf != VMENTER_L1D_FLUSH_NEVER)
>> +	/*
>> +	 * Update static keys and supported arch capabilities according to
>> +	 * the new mitigation state.
>> +	 *
>> +	 * ARCH_CAP_SKIP_VMENTRY_L1DFLUSH is toggled because if we do cache
>> +	 * flushes for L1 guests on (nested) vmlaunch/vmresume to L2, L1
>> +	 * guests can skip the flush and if we don't, then L1 guests need
>> +	 * to do a flush.
>> +	 *
>> +	 * Toggling ARCH_CAP_SKIP_VMENTRY_L1DFLUSH may present inconsistent
>> +	 * model to the guest, e.g., if userspace isn't careful, a VM can
>> +	 * have vCPUs with different values for ARCH_CAPABILITIES. But
>> +	 * there is almost no chance to fix the issue. Because, to present
>> +	 * a consistent model, KVM essentially needs to disallow changing
>> +	 * the module param after VMs/vCPUs have been created, but that
>> +	 * would prevent userspace from toggling the param while VMs are
>> +	 * running, e.g., in response to a new vulnerability.
>> +	 */
>> +	if (l1tf != VMENTER_L1D_FLUSH_NEVER) {
>>   		static_branch_enable(&vmx_l1d_should_flush);
>> -	else
>> +		kvm_caps.supported_arch_cap |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
>> +	} else {
>>   		static_branch_disable(&vmx_l1d_should_flush);
>> +		kvm_caps.supported_arch_cap &= ~ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
>> +	}
>>   	if (l1tf == VMENTER_L1D_FLUSH_COND)
>>   		static_branch_enable(&vmx_l1d_flush_cond);
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c0778ca39650..2408b5f554b7 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1672,7 +1672,7 @@ static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
>>   {
>>   	switch (msr->index) {
>>   	case MSR_IA32_ARCH_CAPABILITIES:
>> -		msr->data = kvm_get_arch_capabilities();
>> +		msr->data = kvm_caps.supported_arch_cap;
>>   		break;
>>   	case MSR_IA32_PERF_CAPABILITIES:
>>   		msr->data = kvm_caps.supported_perf_cap;
>> @@ -7156,7 +7156,7 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>>   			return;
>>   		break;
>>   	case MSR_IA32_TSX_CTRL:
>> -		if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
>> +		if (!(kvm_caps.supported_arch_cap & ARCH_CAP_TSX_CTRL_MSR))
>>   			return;
>>   		break;
>>   	default:
>> @@ -9532,6 +9532,7 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>>   		kvm_caps.max_guest_tsc_khz = max;
>>   	}
>>   	kvm_caps.default_tsc_scaling_ratio = 1ULL << kvm_caps.tsc_scaling_ratio_frac_bits;
>> +	kvm_caps.supported_arch_cap = kvm_get_arch_capabilities();
>>   	kvm_init_msr_lists();
>>   	return 0;
>> @@ -11895,7 +11896,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>>   	if (r)
>>   		goto free_guest_fpu;
>> -	vcpu->arch.arch_capabilities = kvm_get_arch_capabilities();
>> +	vcpu->arch.arch_capabilities = kvm_caps.supported_arch_cap;
>>   	vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
>>   	kvm_xen_init_vcpu(vcpu);
>>   	kvm_vcpu_mtrr_init(vcpu);
>> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>> index c544602d07a3..d3e524bcc169 100644
>> --- a/arch/x86/kvm/x86.h
>> +++ b/arch/x86/kvm/x86.h
>> @@ -29,6 +29,7 @@ struct kvm_caps {
>>   	u64 supported_xcr0;
>>   	u64 supported_xss;
>>   	u64 supported_perf_cap;
>> +	u64 supported_arch_cap;
>>   };
>>   void kvm_spurious_fault(void);
>