linux-kernel - Re: [PATCHv4 1/8] x86/cpu: Add CPUID feature bit for VNMI

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Date:   Thu, 1 Sep 2022 18:15:01 +0530
From:   "Shukla, Santosh" <santosh.shukla@....com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, mlevitsk@...hat.com,
        mail@...iej.szmigiero.name
Subject: Re: [PATCHv4 1/8] x86/cpu: Add CPUID feature bit for VNMI

Hi Jim,

On 9/1/2022 5:12 AM, Jim Mattson wrote:
> On Mon, Aug 29, 2022 at 3:09 AM Santosh Shukla <santosh.shukla@....com> wrote:
>>
>> VNMI feature allows the hypervisor to inject NMI into the guest w/o
>> using Event injection mechanism, The benefit of using VNMI over the
>> event Injection that does not require tracking the Guest's NMI state and
>> intercepting the IRET for the NMI completion. VNMI achieves that by
>> exposing 3 capability bits in VMCB intr_cntrl which helps with
>> virtualizing NMI injection and NMI_Masking.
>>
>> The presence of this feature is indicated via the CPUID function
>> 0x8000000A_EDX[25].
>>
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>> Signed-off-by: Santosh Shukla <santosh.shukla@....com>
>> ---
>>  arch/x86/include/asm/cpufeatures.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index ef4775c6db01..33e3603be09e 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -356,6 +356,7 @@
>>  #define X86_FEATURE_VGIF               (15*32+16) /* Virtual GIF */
>>  #define X86_FEATURE_X2AVIC             (15*32+18) /* Virtual x2apic */
>>  #define X86_FEATURE_V_SPEC_CTRL                (15*32+20) /* Virtual SPEC_CTRL */
>> +#define X86_FEATURE_V_NMI              (15*32+25) /* Virtual NMI */
>>  #define X86_FEATURE_SVME_ADDR_CHK      (15*32+28) /* "" SVME addr check */
> 
> Why is it "V_NMI," but "VGIF"?
> 
I guess you are asking why I chose V_NMI and not VNMI, right?
if so then there are two reasons for going with V_NMI - IP bits are named in order
V_NMI, V_NMI_MASK, and V_NMI_ENABLE style and also Intel already using VNMI (X86_FEATURE_VNMI)

Thanks,
Santosh

> Reviewed-by: Jim Mattson <jmattson@...gle.com>