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Date: Wed, 24 Feb 2016 19:33:15 +0000
From: Matt Fleming <matt@codeblueprint.co.uk>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.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>
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 08:36:33AM, Andy Lutomirski wrote:
> On Wed, Feb 24, 2016 at 8:20 AM, Borislav Petkov <bp@alien8.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.