New "IndustryStandard" fw_cfg?

[Dionna Amalie Glaze]

Hi y'all, I'm Dionna. I work on Confidential VMs at Google Cloud. I've
been keeping up with the TDX and SEV-SNP developments in OVMF and
Linux, and some in Qemu.

There's a new UEFI feature in v2.9 of the specification (March 2021)
that allows for memory ranges to be classified as "unaccepted", since
both TDX and SEV-SNP require that the guest VM accept any host-made
changes to page state. We should expect newer technologies on non-x86
architectures to require memory acceptance as well. Operating systems
are not necessarily going to support this memory type, however.

This leads to a problem: how does the UEFI know that the OS it's going
to boot will support unaccepted memory? Right now we (Google Compute
Engine) have a system of "tagging" for guest image providers to state
that their OS supports some new feature so that we can enable
appropriate configurations for certain images.

I could go about adding a Google-specific fw_cfg file path and
definition to tell our custom OVMF build to use unaccepted memory or
not, but I personally prefer open source. I don't know y'all's process
though, so I'm asking before making a patch set.

There are two approaches I've considered.

1. An arch-specific config key for a u64 value:

The idea would be that I would add QemuFwCfgItemUnacceptedMinimum = 0x8005 here
https://github.com/tianocore/edk2/blob/master/OvmfPkg/Include/IndustryStandard/QemuFwCfg.h#L50

For Qemu, the main code I see for adding config is here, but I'm not
sure what y'all's preferred external configuration method is to get a
value from an invocation (flag, config file, etc) to fw_cfg.c:
https://github.com/qemu/qemu/blob/58b53669e87fed0d70903e05cd42079fbbdbc195/hw/i386/fw_cfg.c#L95

We'd add something like

fw_cfg_add_u64(fw_cfg, FW_CFG_MINIMUM_ACCEPTED_MEMORY_SIZE,
ms->minimum_accepted_memory_size);

where FW_CFG_MINIMUM_ACCEPTED_MEMORY_SIZE is #defined as
FW_CFG_ARCH_LOCAL + 5 in
https://github.com/qemu/qemu/blob/266469947161aa10b1d36843580d369d5aa38589/hw/i386/fw_cfg.h

The name has "minimum" in it since the firmware can choose to accept
more than the minimum, and specifically interpret 0 as UINT64_MAX.

2. A "well-known" file path to be included in the file slots starting
at 0x0020, such as "etc/min_accepted_mem_size", still plumbed through
like in 1.

Thanks!

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