KVM: SVM: Disable AVIC on SNP-enabled system without HvInUseWrAllowed feature

KVM: SVM: Disable AVIC on SNP-enabled system without HvInUseWrAllowed feature

On SNP-enabled system, VMRUN marks AVIC Backing Page as in-use while
the guest is running for both secure and non-secure guest. Any hypervisor
write to the in-use vCPU's AVIC backing page (e.g. to inject an interrupt)
will generate unexpected #PF in the host.

Currently, attempt to run AVIC guest would result in the following error:

    BUG: unable to handle page fault for address: ff3a442e549cc270
    #PF: supervisor write access in kernel mode
    #PF: error_code(0x80000003) - RMP violation
    PGD b6ee01067 P4D b6ee02067 PUD 10096d063 PMD 11c540063 PTE 80000001149cc163
    SEV-SNP: PFN 0x1149cc unassigned, dumping non-zero entries in 2M PFN region: [0x114800 - 0x114a00]
    ...

Newer AMD system is enhanced to allow hypervisor to modify the backing page
for non-secure guest on SNP-enabled system. This enhancement is available
when the CPUID Fn8000_001F_EAX bit 30 is set (HvInUseWrAllowed).

This table describes AVIC support matrix w.r.t. SNP enablement:

               | Non-SNP system |     SNP system
-----------------------------------------------------
 Non-SNP guest |  AVIC Activate | AVIC Activate iff
               |                | HvInuseWrAllowed=1
-----------------------------------------------------
     SNP guest |      N/A       |    Secure AVIC

Therefore, check and disable AVIC in kvm_amd driver when the feature is not
available on SNP-enabled system.

See the AMD64 Architecture Programmer’s Manual (APM) Volume 2 for detail.
(https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/
programmer-references/40332.pdf)

Fixes: 216d106c7ff7 ("x86/sev: Add SEV-SNP host initialization support")
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20241104075845.7583-1-suravee.suthikulpanit@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>