Re: [PATCH v4 14/25] KVM: VMX: Tweak the special handling of SECONDARY_EXEC_ENCLS_EXITING in setup_vmcs_config()

Vitaly Kuznetsov
2022-08-02T12:53:04Z

Sean Christopherson <seanjc@google.com> writes:

> 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

Oh, thanks for this insight, I had no idea! I'll adjust my commit
message accordingly.

>
>> Preserve cpu_has_sgx() check but filter the result of adjust_vmx_controls()
>> instead of the input.
>>
>> Reviewed-by: Jim Mattson <jmattson@google.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.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
>>
>

--
Vitaly