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

[Natarajan, Janakarajan]

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@xxxxxxx>
> > ---
> > 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.