Re: [kvm-unit-tests 02/13] x86: AMD SEV-ES: Setup #VC exception handler for AMD SEV-ES

[Joerg Roedel <jroedel@suse.de>]

Hi Marc,

On Sun, Jan 30, 2022 at 12:36:48PM -0800, Marc Orr wrote:
> Please let me know if I'm mis-understanding this rationale or missing
> any reasons for why folks want a built-in #VC handler.

There are a couple of reasons which come all come down to one goal:
Robustnes of the kvm-unit-tests.

If kvm-unit-tests continue to use the firmware #VC handler after
ExitBootServices there needs to be a contract between the test
framework and the firmware about:

	1) Page-table layout - The page table needs to map the firmware
	   and the shared GHCB used by the firmware.

	2) The firmware is required to keep its #VC handler in the
	   current IDT for kvm-unit-tests to find it and copy the #VC
	   entry into its own IDT.

	3) The firmware #VC handler might use state which is not
	   available anymore after ExitBootServices.

	4) If the firmware uses the kvm-unit-test GHCB after
	   ExitBootServices, it has the get the GHCB address from the
	   GHCB MSR, requiring an identity mapping.
	   Moreover it requires to keep the address of the GHCB in the
	   MSR at all times where a #VC could happen. This could be a
	   problem when we start to add SEV-ES specific tests to the
	   unit-tests, explcitily testing the MSR protocol.

It is easy to violate this implicit protocol and breaking kvm-unit-tests
just by a new version of OVMF being used. I think that is not a very
robust approach and a separate #VC handler in the unit-test framework
makes sense even now.

Regards,

-- 
Jörg Rödel
jroedel@suse.de

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