linux-kernel - Re: [PATCH v2] KVM: nVMX: Sync L2 guest CET states between L1/L2

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Date:   Tue, 02 Mar 2021 11:35:41 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: nVMX: Sync L2 guest CET states between L1/L2

Sean Christopherson <seanjc@...gle.com> writes:

> +Vitaly
>
> On Thu, Feb 25, 2021, Yang Weijiang wrote:
>> These fields are rarely updated by L1 QEMU/KVM, sync them when L1 is trying to
>> read/write them and after they're changed. If CET guest entry-load bit is not
>> set by L1 guest, migrate them to L2 manaully.
>> 
>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> 
>> change in v2:
>>  - Per Sean's review feedback, change CET guest states as rarely-updated fields.
>>    And also migrate L1's CET states to L2 if the entry-load bit is not set.
>>  - Opportunistically removed one blank line.
>> ---
>>  arch/x86/kvm/cpuid.c      |  1 -
>>  arch/x86/kvm/vmx/nested.c | 29 +++++++++++++++++++++++++++++
>>  arch/x86/kvm/vmx/vmx.h    |  3 +++
>>  3 files changed, 32 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 46087bca9418..afc97122c05c 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -143,7 +143,6 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>>  		}
>>  		vcpu->arch.guest_supported_xss =
>>  			(((u64)best->edx << 32) | best->ecx) & supported_xss;
>> -
>>  	} else {
>>  		vcpu->arch.guest_supported_xss = 0;
>>  	}
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 9728efd529a1..1703b8874fad 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -2516,6 +2516,12 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
>>  	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
>>  
>>  	set_cr4_guest_host_mask(vmx);
>> +
>> +	if (kvm_cet_supported()) {
>
> This needs to be conditioned on CET coming from vmcs12, it's on the loading of
> host state on VM-Exit that is unconditional (if CET is supported).
>
> 	if (kvm_cet_supported() && vmx->nested.nested_run_pending &&
> 	    (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {
>
> I also assume these should be guarded by one of the eVMCS fields, though a quick
> search of the public docs didn't provide a hit on the CET fields.
>
> Vitaly, any idea if these will be GUEST_GRP2 or something else?
>

The latest published TLFS I see is 6.0b and it doesn't list anything CET
related in eVMCS v1.0 :-( So I agree with Paolo: we just need to adjust
EVMCS1_UNSUPPORTED_VMENTRY_CTRL/ EVMCS1_UNSUPPORTED_VMEXIT_CTRL for now
and enable it separately later.

>> +		vmcs_writel(GUEST_SSP, vmcs12->guest_ssp);
>> +		vmcs_writel(GUEST_S_CET, vmcs12->guest_s_cet);
>> +		vmcs_writel(GUEST_INTR_SSP_TABLE, vmcs12->guest_ssp_tbl);
>> +	}
>>  }
>

-- 
Vitaly