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

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Message-ID: <20160224193315.GD2603@codeblueprint.co.uk>
Date:	Wed, 24 Feb 2016 19:33:15 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	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>,
	Linus Torvalds <torvalds@...ux-foundation.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 Wed, 24 Feb, at 08:36:33AM, Andy Lutomirski wrote:
> On Wed, Feb 24, 2016 at 8:20 AM, Borislav Petkov <bp@...en8.de> wrote:
> > On Wed, Feb 24, 2016 at 02:10:46PM +0000, Matt Fleming wrote:
> >> > Normally, the only pages with are _PAGE_GLOBAL are those that are in
> >> > the normal kernel mappings (swapper_pg_dir and normal mm_struct pgds).
> >> > By allowing _PAGE_GLOBAL to be set in EFI mappings, you're breaking
> >> > that convention, which forces you to use extra-expensive
> >> > __flush_tlb_all calls in efi_call_virt.
> >
> > Hold on, do you mean the __flush_tlb_all() in the CONFIG_EFI_MIXED code?
> >
> > That's mixed mode. I think you mean the FLUSH_TLB_ALL in efi_call.
> > That's EFI on 64-bit but that is mandated by the spec, AFAIR.
> 
> I mean the one in efi_call_virt.  Why would the spec mandate a TLB
> flush at all?  EFI runtime services have no business touching the
> paging structures directly.  Heck, the 32-bit ones don't even know the
> *format* of the paging structures.
 
Right, and it would necessitate copying out arguments because the
firmware won't understand where/how the kernel has mapped things.

No firmware is going to be doing that.