> > > > 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)
