linux-kernel - Re: [PATCH v3 5/7] KVM: x86: nSVM: implement nested vGIF

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Date:   Mon, 14 Mar 2022 17:21:11 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH v3 5/7] KVM: x86: nSVM: implement nested vGIF

On Wed, 2022-03-09 at 14:40 +0100, Paolo Bonzini wrote:
> The patch is good but I think it's possibly to rewrite some parts in an 
> easier way.
> 
> On 3/1/22 15:36, Maxim Levitsky wrote:
> > +	if (svm->vgif_enabled && (svm->nested.ctl.int_ctl & V_GIF_ENABLE_MASK))
> > +		int_ctl_vmcb12_bits |= (V_GIF_MASK | V_GIF_ENABLE_MASK);
> > +	else
> > +		int_ctl_vmcb01_bits |= (V_GIF_MASK | V_GIF_ENABLE_MASK);
> 
> To remember for later: svm->vmcb's V_GIF_ENABLE_MASK is always the same 
> as vgif:
> 
> - if it comes from vmcb12, it must be 1 (and then vgif is also 1)
> 
> - if it comes from vmcb01, it must be equal to vgif (because 
> V_GIF_ENABLE_MASK is set in init_vmcb and never touched again).
> 
> >   
> > +static bool nested_vgif_enabled(struct vcpu_svm *svm)
> > +{
> > +	if (!is_guest_mode(&svm->vcpu) || !svm->vgif_enabled)
> > +		return false;
> > +	return svm->nested.ctl.int_ctl & V_GIF_ENABLE_MASK;
> > +}
> > +
> >   static inline bool vgif_enabled(struct vcpu_svm *svm)
> >   {
> > -	return !!(svm->vmcb->control.int_ctl & V_GIF_ENABLE_MASK);
> > +	struct vmcb *vmcb = nested_vgif_enabled(svm) ? svm->vmcb01.ptr : svm->vmcb;
> > +
> > +	return !!(vmcb->control.int_ctl & V_GIF_ENABLE_MASK);
> >   }
> >   
> 
> Slight simplification:
> 
> - before the patch, vgif_enabled() is just "vgif", because 
> V_GIF_ENABLE_MASK is set in init_vmcb and copied to vmcb02
> 
> - after the patch, vgif_enabled() is also just "vgif".  Outside guest 
> mode the same reasoning applies.  If L2 has enabled vGIF,  vmcb01's 
> V_GIF_ENABLE is equal to vgif per the previous bullet.  If L2 has not 
> enabled vGIF, vmcb02's V_GIF_ENABLE uses svm->vmcb's int_ctl field which 
> is always the same as vgif (see remark above).
> 
> You can make this simplification a separate patch.


This is a very good idea - I'll do this in a separate patch.

> 
> >  static inline void enable_gif(struct vcpu_svm *svm)
> >  {
> > +	struct vmcb *vmcb = nested_vgif_enabled(svm) ? svm->vmcb01.ptr : svm->vmcb;
> > +
> >  	if (vgif_enabled(svm))
> > -		svm->vmcb->control.int_ctl |= V_GIF_MASK;
> > +		vmcb->control.int_ctl |= V_GIF_MASK;
> >  	else
> >  		svm->vcpu.arch.hflags |= HF_GIF_MASK;
> >  }
> >  
> >  static inline void disable_gif(struct vcpu_svm *svm)
> >  {
> > +	struct vmcb *vmcb = nested_vgif_enabled(svm) ? svm->vmcb01.ptr : svm->vmcb;
> > +
> >  	if (vgif_enabled(svm))
> > -		svm->vmcb->control.int_ctl &= ~V_GIF_MASK;
> > +		vmcb->control.int_ctl &= ~V_GIF_MASK;
> >  	else
> >  		svm->vcpu.arch.hflags &= ~HF_GIF_MASK;
> > +
> >  }
> 
> Looks good.  For a little optimization however you could write
> 
> static inline struct vmcb *get_vgif_vmcb(struct vcpu_svm *svm)
> {
> 	if (!vgif)
> 		return NULL;
> 	if (!is_guest_mode(&svm->vcpu)
> 		return svm->vmcb01.ptr;
> 	if ((svm->vgif_enabled && (svm->nested.ctl.int_ctl & V_GIF_ENABLE_MASK))
> 		return svm->vmcb01.ptr;
> 	else
> 		return svm->nested.vmcb02.ptr;
> }
> 
> and then
> 
> 	struct vmcb *vmcb = get_vgif_vmcb(svm);
> 	if (vmcb)
> 		/* use vmcb->control.int_ctl */
> 	else
> 		/* use hflags */

Good idea as well.

Thanks for the review!
Best regards,
	Maxim Levitsky

> 
> Paolo
> 
> >  
> > +static bool nested_vgif_enabled(struct vcpu_svm *svm)
> > +{
> > +	if (!is_guest_mode(&svm->vcpu) || !svm->vgif_enabled)
> > +		return false;
> > +	return svm->nested.ctl.int_ctl & V_GIF_ENABLE_MASK;
> > +}
> > +