KVM: SEV: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

Implement a workaround for an SNP erratum where the CPU will incorrectly
signal an RMP violation #PF if a hugepage (2MB or 1GB) collides with the
RMP entry of a VMCB, VMSA or AVIC backing page.

When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
backing pages as "in-use" via a reserved bit in the corresponding RMP
entry after a successful VMRUN. This is done for _all_ VMs, not just
SNP-Active VMs.

If the hypervisor accesses an in-use page through a writable
translation, the CPU will throw an RMP violation #PF. On early SNP
hardware, if an in-use page is 2MB-aligned and software accesses any
part of the associated 2MB region with a hugepage, the CPU will
incorrectly treat the entire 2MB region as in-use and signal a an RMP
violation #PF.

To avoid this, the recommendation is to not use a 2MB-aligned page for
the VMCB, VMSA or AVIC pages. Add a generic allocator that will ensure
that the page returned is not 2MB-aligned and is safe to be used when
SEV-SNP is enabled. Also implement similar handling for the VMCB/VMSA
pages of nested guests.

  [ mdr: Squash in nested guest handling from Ashish, commit msg fixups. ]

Reported-by: Alper Gun <[email protected]> # for nested VMSA case
Signed-off-by: Brijesh Singh <[email protected]>
Co-developed-by: Marc Orr <[email protected]>
Signed-off-by: Marc Orr <[email protected]>
Co-developed-by: Ashish Kalra <[email protected]>
Signed-off-by: Ashish Kalra <[email protected]>
Signed-off-by: Michael Roth <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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