linux-kernel - Re: [PATCH v4 14/25] KVM: VMX: Tweak the special handling of SECONDARY_EXEC_ENCLS_EXITING in setup_vmcs

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Date:   Thu, 21 Jul 2022 22:11:29 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 14/25] KVM: VMX: Tweak the special handling of
 SECONDARY_EXEC_ENCLS_EXITING in setup_vmcs_config()

On Thu, Jul 14, 2022, Vitaly Kuznetsov wrote:
> SECONDARY_EXEC_ENCLS_EXITING is conditionally added to the 'optional'
> checklist in setup_vmcs_config() but there's little value in doing so.
> First, as the control is optional, we can always check for its
> presence, no harm done. Second, the only real value cpu_has_sgx() check
> gives is that on the CPUs which support SECONDARY_EXEC_ENCLS_EXITING but
> don't support SGX, the control is not getting enabled. It's highly unlikely
> such CPUs exist but it's possible that some hypervisors expose broken vCPU
> models.

It's not just broken vCPU models, SGX can be "soft-disabled" on bare metal, e.g. if
software writes MCE control MSRs or there's an uncorrectable #MC (may not be the
case on all platforms).  This is architectural behavior and needs to be handled in
KVM.  Obviously if SGX gets disabled after KVM is loaded then we're out of luck, but
having the ENCL-exiting control without SGX being enabled is 100% valid.

As for why KVM bothers with the check, it's to work around a suspected hardware
or XuCode bug (I'm still a bit shocked that's public now :-) ) where SGX got
_hard_ disabled across S3 on some CPUs and made the fields magically disappear.
The workaround was to soft-disable SGX in BIOS so that KVM wouldn't attempt to
enable the ENCLS-exiting control

> Preserve cpu_has_sgx() check but filter the result of adjust_vmx_controls()
> instead of the input.
> 
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ce54f13d8da1..566be73c6509 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2528,9 +2528,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  			SECONDARY_EXEC_PT_CONCEAL_VMX |
>  			SECONDARY_EXEC_ENABLE_VMFUNC |
>  			SECONDARY_EXEC_BUS_LOCK_DETECTION |
> -			SECONDARY_EXEC_NOTIFY_VM_EXITING;
> -		if (cpu_has_sgx())
> -			opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
> +			SECONDARY_EXEC_NOTIFY_VM_EXITING |
> +			SECONDARY_EXEC_ENCLS_EXITING;
> +
>  		if (adjust_vmx_controls(min2, opt2,
>  					MSR_IA32_VMX_PROCBASED_CTLS2,
>  					&_cpu_based_2nd_exec_control) < 0)
> @@ -2577,6 +2577,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  		vmx_cap->vpid = 0;
>  	}
>  
> +	if (!cpu_has_sgx())
> +		_cpu_based_2nd_exec_control &= ~SECONDARY_EXEC_ENCLS_EXITING;
> +
>  	if (_cpu_based_exec_control & CPU_BASED_ACTIVATE_TERTIARY_CONTROLS) {
>  		u64 opt3 = TERTIARY_EXEC_IPI_VIRT;
>  
> -- 
> 2.35.3
>