Re: [PATCH] KVM: VMX: Condition ENCLS-exiting enabling on CPU support for SGX1

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Enable ENCLS-exiting (and thus set vmcs.ENCLS_EXITING_BITMAP) only if
> the CPU supports SGX1.  Per Intel's SDM, all ENCLS leafs #UD if SGX1
> is not supported[*], i.e. intercepting ENCLS to inject a #UD is
> unnecessary.
>
> Avoiding ENCLS-exiting even when it is reported as supported by the CPU
> works around a reported issue where SGX is "hard" disabled after an S3
> suspend/resume cycle, i.e. CPUID.0x7.SGX=0 and the VMCS field/control
> are enumerated as unsupported.  While the root cause of the S3 issue is
> unknown, it's definitely _not_ a KVM (or kernel) bug, i.e. this is a
> workaround for what is most likely a hardware or firmware issue.  As a
> bonus side effect, KVM saves a VMWRITE when first preparing vmcs01 and
> vmcs02.
>
> Query CPUID directly instead of going through cpu_data() or cpu_has() to
> ensure KVM is trapping ENCLS when it's supported in hardware, e.g. even
> if X86_FEATURE_SGX1 (which doesn't yet exist in upstream) were disabled
> by the kernel/user.

I would prefer this paragraph to become a comment right above
cpu_has_sgx() or I bet we'll be getting a lot of 'avoid open-coding
boot_cpu_has() ...' patches in the future.

>
> Note, SGX must be disabled in BIOS to take advantage of this workaround
>
> [*] The additional ENCLS CPUID check on SGX1 exists so that SGX can be
>     globally "soft" disabled post-reset, e.g. if #MC bits in MCi_CTL are
>     cleared.  Soft disabled meaning disabling SGX without clearing the
>     primary CPUID bit (in leaf 0x7) and without poking into non-SGX
>     CPU paths, e.g. for the VMCS controls.
>
> Fixes: 0b665d304028 ("KVM: vmx: Inject #UD for SGX ENCLS instruction in guest")
> Reported-by: Toni Spets <toni.spets@iki.fi>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>
> This seems somewhat premature given that we don't yet know if the observed
> behavior is a logic bug, a one off manufacturing defect, firmware specific,
> etc...  On the other hand, the change is arguably an optimization
> irrespective of using it as a workaround.
>
>  arch/x86/kvm/vmx/vmx.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e6138cd5..50cab98382e7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2338,6 +2338,11 @@ static void hardware_disable(void)
>  	kvm_cpu_vmxoff();
>  }
>  
> +static bool cpu_has_sgx(void)
> +{
> +	return cpuid_eax(0) >= 0x12 && (cpuid_eax(0x12) & BIT(0));
> +}
> +
>  static __init int adjust_vmx_controls(u32 ctl_min, u32 ctl_opt,
>  				      u32 msr, u32 *result)
>  {
> @@ -2418,8 +2423,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  			SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE |
>  			SECONDARY_EXEC_PT_USE_GPA |
>  			SECONDARY_EXEC_PT_CONCEAL_VMX |
> -			SECONDARY_EXEC_ENABLE_VMFUNC |
> -			SECONDARY_EXEC_ENCLS_EXITING;
> +			SECONDARY_EXEC_ENABLE_VMFUNC;
> +		if (cpu_has_sgx())
> +			opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
>  		if (adjust_vmx_controls(min2, opt2,
>  					MSR_IA32_VMX_PROCBASED_CTLS2,
>  					&_cpu_based_2nd_exec_control) < 0)

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly