x86/apic: Populate .read()/.write() callbacks of Secure AVIC driver

x86/apic: Populate .read()/.write() callbacks of Secure AVIC driver

Add read() and write() APIC callback functions to read and write the x2APIC
registers directly from the guest APIC backing page of a vCPU.

The x2APIC registers are mapped at an offset within the guest APIC backing
page which is the 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.

When Secure AVIC is enabled, accessing the guest's APIC registers through
RD/WRMSR results in a #VC exception (for non-accelerated register accesses)
with error code VMEXIT_AVIC_NOACCEL.

The #VC exception handler can read/write the x2APIC register in the guest APIC
backing page to complete the RDMSR/WRMSR. Since doing this would increase the
latency of accessing the x2APIC registers, instead of doing RDMSR/WRMSR based
register accesses and handling reads/writes in the #VC exception, directly
read/write the APIC registers from/to the guest APIC backing page of the vCPU
in read() and write() callbacks of the Secure AVIC APIC driver.

  [ bp: Massage commit message. ]

Co-developed-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Link: https://lore.kernel.org/20250828110255.208779-1-Neeraj.Upadhyay@amd.com