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

["Melody (Huibo) Wang" <huibo.wang@amd.com>]

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. 

Thanks,
Melody