[edk2-devel] [PATCH 07/14] OvmfPkg: Add PCD and DEFINEs for Lazy Accept page.

[Dionna Glaze via groups.io]

>
>
> > With the help of fw_cfg, we can know the size of
> > QemuFwCfgItemKernelSize/
> > QemuFwCfgItemInitrdSize/QemuFwCfgItemKernelSetupSize.
>
> That'll only work with direct kernel boot, when some boot loader
> is in your boot workflow this will not work.

Indeed, the TDX startup library does things very differently from how
we'd handle this for SEV-SNP. There are already 2 ranges of memory
prevalidated in SEC that have to be specially skipped over in PEI.
I tried the lazy accept changes from edk2-staging with SEV-SNP changes
to prevalidate only the PEI installed memory
https://github.com/AMDESE/ovmf/pull/4 and I just could not manage to
get the guest to boot. It'd crash in the QemuTryBootKernel part of
BDS.

Tom Lendacky's suggestion for SEV-SNP is to pre-accept all memory
under 4GB to make all that complexity go away. Only this approach
worked in my own testing. With the MMIO hole it's just validating 3GB
of memory.

>
> > > For guests which don't support lazy accept we could offer a
> > > (runtime) option to simply accept all memory in ExitBootServices.
> > What is the runtime option? Some new fw_cfg?
>
> Ideally without manual configuration (see also the reply in Dionna's thread).
>
> I think ovmf should start in lazy accept mode unconditionally.  Accept
> enough memory to reach dxe, but not more.  Accept additional memory
> needed on demand.

This will probably be complicated for SEV-SNP. I couldn't get it to
work as discussed above, but I wouldn't count myself a UEFI expert.

>
> Whenever we pass unaccepted memory to the guest os or not can be decided
> rather late, I think we can wait with that until the first
> BS->GetMemoryMap() call comes in.  Doing it late gives is more options
> to handle things automatically and it also simplifies SEC code.
>
> take care,
>   Gerd
>


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


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