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 09:35:28 +0700
From:   "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To:     Maxim Levitsky <mlevitsk@...hat.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 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.

Best Regards,
Suravee