Version 2 of GHCB specification added support for the SNP Guest Request
Message NAE event. The event allows for an SEV-SNP guest to make
requests to the SEV-SNP firmware through the hypervisor using the
SNP_GUEST_REQUEST API defined in the SEV-SNP firmware specification.

This is used by guests primarily to request attestation reports from
firmware. There are other request types are available as well, but the
specifics of what guest requests are being made generally does not
affect how they are handled by the hypervisor, which only serves as a
proxy for the guest requests and firmware responses.

Implement handling for these events.

When an SNP Guest Request is issued, the guest will provide its own
request/response pages, which could in theory be passed along directly
to firmware. However, these pages would need special care:

  - Both pages are from shared guest memory, so they need to be
    protected from migration/etc. occurring while firmware reads/writes
    to them. At a minimum, this requires elevating the ref counts and
    potentially needing an explicit pinning of the memory. This places
    additional restrictions on what type of memory backends userspace
    can use for shared guest memory since there would be some reliance
    on using refcounted pages.

  - The response page needs to be switched to Firmware-owned state
    before the firmware can write to it, which can lead to potential
    host RMP #PFs if the guest is misbehaved and hands the host a
    guest page that KVM is writing to for other reasons (e.g. virtio
    buffers).

Both of these issues can be avoided completely by using
separately-allocated bounce pages for both the request/response pages
and passing those to firmware instead. So that's the approach taken
here.
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Co-developed-by: Alexey Kardashevskiy <aik@amd.com>
Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Co-developed-by: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
[mdr: ensure FW command failures are indicated to guest, drop extended
 request handling to be re-written as separate patch, massage commit]
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240701223148.3798365-2-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>