linux-kernel - Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page table mappings

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Message-ID: <20160302112036.GB2649@codeblueprint.co.uk>
Date:	Wed, 2 Mar 2016 11:20:36 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Sylvain Chouleur <sylvain.chouleur@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Toshi Kani <toshi.kani@...com>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Luis Rodriguez <mcgrof@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, ricardo.neri@...el.com,
	Hugh Dickins <hughd@...gle.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page
 table mappings

On Mon, 29 Feb, at 11:56:56AM, Sylvain Chouleur wrote:
> 2016-02-24 20:45 GMT+01:00 Matt Fleming <matt@...eblueprint.co.uk>:
> > On Wed, 24 Feb, at 10:56:13AM, Linus Torvalds wrote:
> >> On Wed, Feb 24, 2016 at 8:36 AM, Andy Lutomirski <luto@...capital.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
> 
> I was thinking we could use the efi kthread to handle the efi services
> generically, not only for the interruptible case, and have a way to decide if we
> allow interruptions inside the efi call itself or not.
> 
> Then all runtime services would use an mm_struct. The drawback is that you will
> need two context switchs to be able to execute the runtime service.

I would be surprised if the asynchronous nature of having a special
EFI kthread would buy you any benefit in general. And in fact, in the
efi-pstore code you can be invoked in IRQ context and you really don't
want to start talking to a kthread.