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

[Neeraj Upadhyay]

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