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

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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
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[PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME regions Matt Fleming <matt@codeblueprint.co.uk> - 2015-09-26 00:10 +0200
  Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Ingo Molnar <mingo@kernel.org> - 2015-09-26 08:10 +0200
    Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME regions Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2015-09-26 09:10 +0200
      Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Ingo Molnar <mingo@kernel.org> - 2015-09-27 09:10 +0200
        Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Borislav Petkov <bp@alien8.de> - 2015-09-27 12:20 +0200
          Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Ingo Molnar <mingo@kernel.org> - 2015-09-28 08:30 +0200
          Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Dave Young <dyoung@redhat.com> - 2015-09-29 11:40 +0200
            Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Borislav Petkov <bp@alien8.de> - 2015-09-29 12:30 +0200
          Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Matt Fleming <matt@codeblueprint.co.uk> - 2015-09-29 16:40 +0200
            Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions "H. Peter Anvin" <hpa@zytor.com> - 2015-09-30 03:00 +0200
              Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Borislav Petkov <bp@alien8.de> - 2015-09-30 10:40 +0200
        Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions "H. Peter Anvin" <hpa@zytor.com> - 2015-09-30 03:10 +0200
          Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME regions Andy Lutomirski <luto@amacapital.net> - 2015-09-30 03:20 +0200
            Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions "H. Peter Anvin" <hpa@zytor.com> - 2015-09-30 03:20 +0200
            Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME regions Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2015-09-30 06:30 +0200
          Re: [PATCH 2/2] arm64/efi: Don't pad between EFI_MEMORY_RUNTIME  regions Ingo Molnar <mingo@kernel.org> - 2015-10-01 12:50 +0200
  [tip:core/urgent] arm64/efi:   Fix boot crash by not padding between EFI_MEMORY_RUNTIME regions tip-bot for Ard Biesheuvel <tipbot@zytor.com> - 2015-10-01 15:00 +0200

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