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EFI is basically our existing embodiment of this fw/kernel negotiation
thing, and iff we need it, I have no objection to using it for this
purpose, i.e., to allow the firmware to infer whether or not it should
accept all available memory on behalf of the OS before exiting boot
services. But if we don't need this, even better.

FW/kernel negotiation does not work if there's a boot loader in the middle
that does ExitBootServices(). By the time kernel can announce if it
supports unaccepted memory there's nobody to announce to.

Why would you want to support such bootloaders for TDX anyway? TDX
heavily relies on measured boot abstractions and other things that are
heavily tied to firmware.

I don't understand it either. And, yet, there's demand for it.

I think there's no good solution for this bad upgrade path that the
UEFI spec stuck us with, so I think I'm going to stick to what many
folks have suggested: just have the host require external information.
What this means is that at VM creation time, the user has to specify
an extra flag that all memory has to be accepted in firmware before
booting the guest OS. Failure to provide the flag leads to the
unfortunate outcome that the VM only has access to the lower 4GB of
RAM. We can only hope that the VM OOMs shortly after they start up the
machine and the user reads an FAQ that they should add this flag.

I'll do a round of appeals to distributions to include this patch set
and AMD's follow-up that defines accept_memory for SEV-SNP to reduce
the time that people need to know about this flag.

-- 
-Dionna Glaze, PhD (she/her)