KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event

KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event

Version 2 of GHCB specification added support for the SNP Extended Guest
Request Message NAE event. This event serves a nearly identical purpose
to the previously-added SNP_GUEST_REQUEST event, but for certain message
types it allows the guest to supply a buffer to be used for additional
information in some cases.

Currently the GHCB spec only defines extended handling of this sort in
the case of attestation requests, where the additional buffer is used to
supply a table of certificate data corresponding to the attestion
report's signing key. Support for this extended handling will require
additional KVM APIs to handle coordinating with userspace.

Whether or not the hypervisor opts to provide this certificate data is
optional. However, support for processing SNP_EXTENDED_GUEST_REQUEST
GHCB requests is required by the GHCB 2.0 specification for SNP guests,
so for now implement a stub implementation that provides an empty
certificate table to the guest if it supplies an additional buffer, but
otherwise behaves identically to SNP_GUEST_REQUEST.

Reviewed-by: Carlos Bilbao <carlos.bilbao.osdev@gmail.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240701223148.3798365-4-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>