Don't clear the C-bit in the #NPF handler, as it is a legal GPA bit for
non-SEV guests, and for SEV guests the C-bit is dropped before the GPA
hits the NPT in hardware.  Clearing the bit for non-SEV guests causes KVM
to mishandle #NPFs with that collide with the host's C-bit.

Although the APM doesn't explicitly state that the C-bit is not reserved
for non-SEV, Tom Lendacky confirmed that the following snippet about the
effective reduction due to the C-bit does indeed apply only to SEV guests.

  Note that because guest physical addresses are always translated
  through the nested page tables, the size of the guest physical address
  space is not impacted by any physical address space reduction indicated
  in CPUID 8000_001F[EBX]. If the C-bit is a physical address bit however,
  the guest physical address space is effectively reduced by 1 bit.

And for SEV guests, the APM clearly states that the bit is dropped before
walking the nested page tables.

  If the C-bit is an address bit, this bit is masked from the guest
  physical address when it is translated through the nested page tables.
  Consequently, the hypervisor does not need to be aware of which pages
  the guest has chosen to mark private.

Note, the bogus C-bit clearing was removed from legacy #PF handler in
commit 6d1b867d ("KVM: SVM: Don't strip the C-bit from CR2 on #PF
interception").

Fixes: 0ede79e1 ("KVM: SVM: Clear C-bit from the page fault address")
Cc: Peter Gonda <pgonda@google.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210625020354.431829-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>