linux-kernel - Re: [RFC 03/14] x86/apic: Populate .read()/.write() callbacks of Secure AVIC driver

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Message-ID: <599c8b26-0a9c-4ed2-9e35-be51db27a6f4@amd.com>
Date: Thu, 7 Nov 2024 09:03:22 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: "Melody (Huibo) Wang" <huibo.wang@....com>, linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 Thomas.Lendacky@....com, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, bp@...en8.de,
 David.Kaplan@....com, x86@...nel.org, hpa@...or.com, peterz@...radead.org,
 seanjc@...gle.com, pbonzini@...hat.com, kvm@...r.kernel.org
Subject: Re: [RFC 03/14] x86/apic: Populate .read()/.write() callbacks of
 Secure AVIC driver



On 11/7/2024 12:50 AM, Melody (Huibo) Wang wrote:
> Hi Neeraj,
> 
> On 9/13/2024 4:36 AM, Neeraj Upadhyay wrote:
>> The x2APIC registers are mapped at an offset within the guest APIC
>> backing page which is same as their x2APIC MMIO offset. Secure AVIC
>> adds new registers such as ALLOWED_IRRs (which are at 4-byte offset
>> within the IRR register offset range) and NMI_REQ to the APIC register
>> space. In addition, the APIC_ID register is writable and configured by
>> guest.
>>
>> Add read() and write() APIC callback functions to read and write x2APIC
>> registers directly from the guest APIC backing page.
>>
>> The default .read()/.write() callbacks of x2APIC drivers perform
>> a rdmsr/wrmsr of the x2APIC registers. When Secure AVIC is enabled,
>> these would result in #VC exception (for non-accelerated register
>> accesses). The #VC exception handler reads/write the x2APIC register
>> in the guest APIC backing page. Since this would increase the latency
>> of accessing x2APIC registers, the read() and write() callbacks of
>> Secure AVIC driver directly reads/writes to the guest APIC backing page.
>>
> I think this is important non-obvious information which should be in a comment in the code
> itself, not just in the commit message. 
> 

Sure, I will add some of this information in the comments. Thanks for the review!


- Neeraj

> Thanks,
> Melody