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

[Matt Fleming <matt@codeblueprint.co.uk>]

On Sun, 27 Sep, at 12:40:14PM, Borislav Petkov wrote:
> On Sun, Sep 27, 2015 at 09:06:44AM +0200, Ingo Molnar wrote:
> > 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.
> 
> The main reason why we did the additional, top-down mapping was kexec
> kernel wanting to use UEFI runtime facilities too and the braindead
> design of SetVirtualAddressMap() being callable only once per system
> boot. So we had to have stable mappings which are valid in the kexec-ed
> kernel too.
> 
> But this was long time ago and I most certainly have forgotten all the
> details.
 
That's a pretty good summary for x86. I think specifically the reason
we map the EFI memmap entries "backwards" (entry N has higher VA than
entry N+1) is because the code was easier to write that way, but
you'll know better than me ;-)

> And now I'm wondering why didn't we do the 1:1 thing and rebuild the
> exact same EFI pagetable in the kexec-ed kernel? Because when we do
> an EFI call, we switch to the special pagetable so why didn't we make
> the kexec-ed kernel rebuild the 1:1 pagetable which it can use for EFI
> calls...
> 
> Hmm, again, I've forgotten a lot of details so I'm sure Matt will come
> in and say "No, you can't do that because..."

I *think* the only reason was the Apple firmware problem where it
explodes if you pass the 1:1 mappings to SetVirtualAddressMap(). And
obviously people do want to use kexec with Apple machines.

It's probably worth revisiting this whole thing from the x86 side.

-- 
Matt Fleming, Intel Open Source Technology Center