linux-kernel - Re: [PATCH v3 3/3] KVM: SVM: Extend host physical APIC ID field to support more than 8-bit

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Message-ID: <45c0c1c2-f660-afb4-9631-e73cbbe60465@amd.com>
Date:   Tue, 1 Feb 2022 21:56:55 +0700
From:   "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        pbonzini@...hat.com, joro@...tes.org, mlevitsk@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        peterz@...radead.org, hpa@...or.com, thomas.lendacky@....com,
        jon.grimm@....com
Subject: Re: [PATCH v3 3/3] KVM: SVM: Extend host physical APIC ID field to
 support more than 8-bit

Sean,

On 12/31/2021 12:21 AM, Sean Christopherson wrote:
> On Mon, Dec 13, 2021, Suravee Suthikulpanit wrote:
>> The AVIC physical APIC ID table entry contains the host physical
>> APIC ID field, which the hardware uses to keep track of where each
>> vCPU is running. Originally, the field is an 8-bit value, which can
>> only support physical APIC ID up to 255.
>>
>> To support system with larger APIC ID, the AVIC hardware extends
>> this field to support up to the largest possible physical APIC ID
>> available on the system.
>>
>> Therefore, replace the hard-coded mask value with the value
>> calculated from the maximum possible physical APIC ID in the system.
> 
> ...
> 
>> +static void avic_init_host_physical_apicid_mask(void)
>> +{
>> +	if (!x2apic_mode) {
>> +		/* If host is in xAPIC mode, default to only 8-bit mask. */
>> +		avic_host_physical_id_mask = 0xffULL;
> 
> Use GENMASK(7:0) instead of open coding the equivalent.  Oh, and
> avic_host_physical_id_mask doesn't need to be a u64, it's hard capped at 12 bits
> and so can be a simple int.
> 

Actually, shouldn't it be u16 since the value returned from kvm_cpu_get_apicid()
would typically be 16-bit value (despite it has int as a return type).

Regards,
Suravee