linux-kernel - Re: [PATCH 2/2] KVM: SVM: Enable Virtual GIF feature

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Date:   Wed, 16 Aug 2017 17:04:10 -0500
From:   "Natarajan, Janakarajan" <Janakarajan.Natarajan@....com>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Ghannam, Yazen" <Yazen.Ghannam@....com>
Subject: Re: [PATCH 2/2] KVM: SVM: Enable Virtual GIF feature

On 8/16/2017 2:53 PM, Radim Krcmar wrote:
> 2017-08-16 10:54-0500, Janakarajan Natarajan:
>> Enable the Virtual GIF feature. This is done by setting bit 25 at position
>> 60h in the vmcb.
>>
>> With this feature enabled, the processor uses bit 9 at position 60h as the
>> virtual GIF when executing STGI/CLGI instructions.
>>
>> Since the execution of STGI by the L1 hypervisor does not cause a return to
>> the outermost (L0) hypervisor, the enable_irq_window and enable_nmi_window
>> are modified.
>>
>> The IRQ and NMI windows will be opened even if GIF is not set, under the
>> assumption that on resuming the L1 hypervisor the IRQ and NMI will be
>> held pending until the processor executes the STGI instruction.
>>
>> Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@....com>
>> ---
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> @@ -4702,8 +4735,8 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
>>   	    == HF_NMI_MASK)
>>   		return; /* IRET will cause a vm exit */
>>   
>> -	if ((svm->vcpu.arch.hflags & HF_GIF_MASK) == 0)
>> -		return; /* STGI will cause a vm exit */
>> +	if (!vgif_enabled(svm) && !gif_set(svm))
>> +		return; /* STGI will cause a vm exit or HW will set VGIF*/
> Why don't we enable STGI interception to get notified that the window
> has opened?  (I doubt that single stepping would be faster ...)

It would defeat the purpose of having vGIF - execute STGI/CLGI and not 
have a world-switch. Plus
it would be like implementing the vGIF feature halfway, allowing only 
CLGI to take advantage of the
hardware.

>
> Thanks.