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SubjectRe: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME regions

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 09/27/2015 12:06 AM, Ingo Molnar wrote:
> >
> > * Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >
> >>> If we allocate the EFI runtime as a single virtual memory block then issues
> >>> like rounding between sections does not even come up as a problem: we map the
> >>> original offsets and sizes byte by byte.
> >>
> >> Well, by that reasoning, we should not call SetVirtualAddressMap() in the first
> >> place, and just use the 1:1 mapping UEFI uses natively. This is more than
> >> feasible on arm64, and I actually fought hard against using
> >> SetVirtualAddressMap() at all, but I was overruled by others. I think this is
> >> also trivially possible on X64, since the 1:1 mapping is already active
> >> alongside the VA mapping.
> >
> > Could we please re-list all the arguments pro and contra of 1:1 physical mappings,
> > in a post that also explains the background so that more people can chime in, not
> > just people versed in EFI internals? It's very much possible that a bad decision
> > was made.
> >
>
> Pro: by far the sanest way to map the UEFI tables.
> Con: doesn't actually work (breaks on several known platforms.)

You knew this next question was coming: in what way does it break on known
platforms?

I.e. do those platforms require a SetVirtualAddressMap() call and break if one
does not come?

Note that there's 3 models possible:

- pure 1:1
- 1:1 plus offset, with SetVirtualAddressMap(offset)
- bottom up allocator

I don't think we want 'pure' 1:1 physical/virtual (for security reasons, etc.).

So the question is, in what way does our current proposed bottom-up allocator
differ from 1:1 plus offset? My impression is that they are mostly identical.

Thanks,

Ingo


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