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Date: Wed, 24 Feb 2016 19:45:01 +0000
From: Matt Fleming <matt@codeblueprint.co.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>, Borislav Petkov <bp@alien8.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Toshi Kani <toshi.kani@hp.com>, Brian Gerst <brgerst@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Luis Rodriguez <mcgrof@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
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        Peter Zijlstra <peterz@infradead.org>, ricardo.neri@intel.com,
        Hugh Dickins <hughd@google.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Sylvain Chouleur <sylvain.chouleur@gmail.com>
Subject: Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page
 table mappings
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On Wed, 24 Feb, at 10:56:13AM, Linus Torvalds wrote:
> On Wed, Feb 24, 2016 at 8:36 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> >>
> >> So the EFI runtime crap should not change once it is mapped. And those
> >> should be global. It is only natural.
> >
> > Why is it natural?
> >
> > Long-term, I'd rather see EFI runtime services use an actual mm_struct
> > and use_mm.
> 
> Definitely.
> 
> The EFI runtime page mapping may be unchanging, but that doesn't mean
> we should be mapping it all the time - the mapping may not change, but
> we will change away from it.
 
There is movement towards hanging the EFI memory map off of mm_struct
for x86. ARM and arm64 already do this and there were some patches
from Sylvain (Cc'd) to do this for the purposes of having a task
context that could be preempted while in the middle of an EFI runtime
call for some Intel platforms,

  https://lkml.kernel.org/r/1452702762-27216-4-git-send-email-sylvain.chouleur@gmail.com

Apart from the code simplification and not being required to open-code
the %cr3 diddling, are there other benefits of mm_struct and use_mm()
that make it appealing in the non-preemptible case?

Not that those aren't reasons enough.

> So marking those pages global is very wrong.

Ingo, Andy, how do you want to handle this patch? Maybe just drop it
from tip/efi/core while we prod around making all the EFI mappings
non-global? Nothing else depends on it, it can be dropped without any
harm.