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

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Date:	Wed, 24 Feb 2016 11:50:14 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Sylvain Chouleur <sylvain.chouleur@...il.com>
Subject: Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page
 table mappings

On Wed, Feb 24, 2016 at 11:45 AM, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> 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
>
> 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.

If we add PCID support, then use_mm will get the benefits (~200ns
savings for a round trip) for free.

>
>> 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.

If the patch is harmless as is, I'm okay with letting it stay.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC