linux-kernel - Re: [PATCH v2 07/11] KVM: x86: add a delayed hardware NMI injection interface

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Date:   Fri, 24 Feb 2023 16:39:10 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Sandipan Das <sandipan.das@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Babu Moger <babu.moger@....com>, linux-kernel@...r.kernel.org,
        Jing Liu <jing2.liu@...el.com>,
        Wyes Karny <wyes.karny@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 07/11] KVM: x86: add a delayed hardware NMI injection
 interface

On Sat, 2023-01-28 at 01:09 +0000, Sean Christopherson wrote:
> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> > This patch adds two new vendor callbacks:
> 
> No "this patch" please, just say what it does.
> 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 684a5519812fb2..46993ce61c92db 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -871,8 +871,13 @@ struct kvm_vcpu_arch {
> >  	u64 tsc_scaling_ratio; /* current scaling ratio */
> >  
> >  	atomic_t nmi_queued;  /* unprocessed asynchronous NMIs */
> > -	unsigned nmi_pending; /* NMI queued after currently running handler */
> > +
> > +	unsigned int nmi_pending; /*
> > +				   * NMI queued after currently running handler
> > +				   * (not including a hardware pending NMI (e.g vNMI))
> > +				   */
> 
> Put the block comment above.  I'd say collapse all of the comments about NMIs into
> a single big block comment.
> 
> >  	bool nmi_injected;    /* Trying to inject an NMI this entry */
> > +
> >  	bool smi_pending;    /* SMI queued after currently running handler */
> >  	u8 handling_intr_from_guest;
> >  
> > @@ -10015,13 +10022,34 @@ static void process_nmi(struct kvm_vcpu *vcpu)
> >  	 * Otherwise, allow two (and we'll inject the first one immediately).
> >  	 */
> >  	if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
> > -		limit = 1;
> > +		limit--;
> > +
> > +	/* Also if there is already a NMI hardware queued to be injected,
> > +	 * decrease the limit again
> > +	 */
> 
> 	/*
> 	 * Block comment ...
> 	 */
> 
> > +	if (static_call(kvm_x86_get_hw_nmi_pending)(vcpu))
> 
> I'd prefer "is_hw_nmi_pending()" over "get", even if it means not pairing with
> "set".  Though I think that's a good thing since they aren't perfect pairs.
> 
> > +		limit--;
> >  
> > -	vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> > +	if (limit <= 0)
> > +		return;
> > +
> > +	/* Attempt to use hardware NMI queueing */
> > +	if (static_call(kvm_x86_set_hw_nmi_pending)(vcpu)) {
> > +		limit--;
> > +		nmi_to_queue--;
> > +	}
> > +
> > +	vcpu->arch.nmi_pending += nmi_to_queue;
> >  	vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
> >  	kvm_make_request(KVM_REQ_EVENT, vcpu);
> >  }
> >  
> > +/* Return total number of NMIs pending injection to the VM */
> > +int kvm_get_total_nmi_pending(struct kvm_vcpu *vcpu)
> > +{
> > +	return vcpu->arch.nmi_pending + static_call(kvm_x86_get_hw_nmi_pending)(vcpu);
> 
> Nothing cares about the total count, this can just be;

I wanted to have the interface to be a bit more generic so that in theory you could have
more that one hardware NMI pending. I don't care much about it.


Best regards,
	Maxim Levitsky

> 
> 
> 	bool kvm_is_nmi_pending(struct kvm_vcpu *vcpu)
> 	{
> 		return vcpu->arch.nmi_pending ||
> 		       static_call(kvm_x86_is_hw_nmi_pending)(vcpu);
> 	}
> 
> 
> > +}
> > +
> >  void kvm_make_scan_ioapic_request_mask(struct kvm *kvm,
> >  				       unsigned long *vcpu_bitmap)
> >  {
> > -- 
> > 2.26.3
> >