linux-kernel - Re: [PATCH v6 15/17] KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible

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Date:   Tue, 28 Jun 2022 11:59:55 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com, joro@...tes.org, jon.grimm@....com,
        wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v6 15/17] KVM: SVM: Use target APIC ID to complete
 x2AVIC IRQs when possible

On Tue, 2022-06-28 at 09:35 +0700, Suthikulpanit, Suravee wrote:
> 
> On 6/28/2022 5:55 AM, Maxim Levitsky wrote:
> > On Fri, 2022-06-24 at 18:41 +0200, Paolo Bonzini wrote:
> > > On 5/19/22 12:27, Suravee Suthikulpanit wrote:
> > > > +			 * If the x2APIC logical ID sub-field (i.e. icrh[15:0]) contains zero
> > > > +			 * or more than 1 bits, we cannot match just one vcpu to kick for
> > > > +			 * fast path.
> > > > +			 */
> > > > +			if (!first || (first != last))
> > > > +				return -EINVAL;
> > > > +
> > > > +			apic = first - 1;
> > > > +			if ((apic < 0) || (apic > 15) || (cluster >= 0xfffff))
> > > > +				return -EINVAL;
> > > 
> > > Neither of these is possible: first == 0 has been cheked above, and
> > > ffs(icrh & 0xffff) cannot exceed 15.  Likewise, cluster is actually
> > > limited to 16 bits, not 20.
> > > 
> > > Plus, C is not Pascal so no parentheses. :)
> > > 
> > > Putting everything together, it can be simplified to this:
> > > 
> > > +                       int cluster = (icrh & 0xffff0000) >> 16;
> > > +                       int apic = ffs(icrh & 0xffff) - 1;
> > > +
> > > +                       /*
> > > +                        * If the x2APIC logical ID sub-field (i.e. icrh[15:0])
> > > +                        * contains anything but a single bit, we cannot use the
> > > +                        * fast path, because it is limited to a single vCPU.
> > > +                        */
> > > +                       if (apic < 0 || icrh != (1 << apic))
> > > +                               return -EINVAL;
> > > +
> > > +                       l1_physical_id = (cluster << 4) + apic;
> > > 
> > > 
> > > > +			apic_id = (cluster << 4) + apic;
> > 
> > Hi Paolo and Suravee Suthikulpanit!
> > 
> > Note that this patch is not needed anymore, I fixed the avic_kick_target_vcpus_fast function,
> > and added the support for x2apic because it was very easy to do
> > (I already needed to parse logical id for flat and cluser modes)
> > 
> > Best regards,
> > 	Maxim Levitsky
> > 
> 
> Understood. I was about to send v7 to remove this patch from the series, but too late. I'll test the current queue branch and provide update.

Also this really needs a KVM unit test, to avoid breaking corner cases like
sending IPI to 0xFF address, which was the reason I had to fix the 
avic_kick_target_vcpus_fast.

We do have 'apic' test in kvm unit tests,
and I was already looking to extend it to cover more cases and to run it with AVIC's
compatible settings. I hope I will be able to do this this week.

Best regards,
	Maxim Levitsky


> 
> Best Regards,
> Suravee
>