linux-kernel - Re: [PATCH v2 16/28] KVM: VMX: Tweak the special handling of SECONDARY_EXEC_ENCLS_EXITING in setup_vmcs

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Message-ID: <87wncysj1k.fsf@redhat.com>
Date:   Thu, 30 Jun 2022 09:32:39 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 16/28] KVM: VMX: Tweak the special handling of
 SECONDARY_EXEC_ENCLS_EXITING in setup_vmcs_config()

Jim Mattson <jmattson@...gle.com> writes:

> On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@...hat.com> 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.
>>
>> Preserve cpu_has_sgx() check but filter the result of adjust_vmx_controls()
>> instead of the input.
>>
>> 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 89a3bbafa5af..e32d91006b80 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;
>
> NYC, but why is there a leading underscore here?

No idea to be honest, this goes way back to 2007 when
setup_vmcs_config() was introduced:

commit 1c3d14fe0ab75337a3f6c06b6bc18bcbc2b3d0bc
Author: Yang, Sheng <sheng.yang@...el.com>
Date:   Sun Jul 29 11:07:42 2007 +0300

    KVM: VMX: Improve the method of writing vmcs control

>
>>         if (_cpu_based_exec_control & CPU_BASED_ACTIVATE_TERTIARY_CONTROLS) {
>>                 u64 opt3 = TERTIARY_EXEC_IPI_VIRT;
>>
>> --
>> 2.35.3
>>
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
>

Thanks!

-- 
Vitaly