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

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Date:   Mon, 1 Mar 2021 09:46:19 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2] KVM: nVMX: Sync L2 guest CET states between L1/L2

+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?

> +		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);
> +	}
>  }