linux-kernel - Re: [BUG] x86/efi: MMRs no longer properly mapped after switch to isolated page table

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Date:	Thu, 28 Apr 2016 14:57:11 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-efi@...r.kernel.org, Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>,
	mike travis <travis@....com>, Nathan Zimmer <nzimmer@....com>
Subject: Re: [BUG] x86/efi: MMRs no longer properly mapped after switch to
 isolated page table

On Wed, Apr 27, 2016 at 08:41:28PM -0500, Alex Thorlton wrote:
> In this particular instance, it's not using the EFI page table - it's
> showing that the register isn't mapped into the main kernel page table,
> via the bad paging request.  The issue isn't that there's something
> wrong with the EFI page table, but that something appears to be missing
> from the kernel table.

So my question stands: you said 67a9108ed4313 is causing this and this
commit creates an EFI-specific page table and that shouldn't have
anything to do with how the MMR stuff is mapped, should it?

> Well, quite a while back, these MMRs got mapped in using
> init_extra_mapping_uc in map_low_mmrs.  Back then, yes, they were mapped
> straight into the kernel page table.
> 
> After d2f7cbe7b26a74 ("x86/efi: Runtime services virtual mapping") was
> introduced (I'm sure you remember the BIOS issue we had a while back) we
> had to fall back to using EFI_OLD_MEMMAP, so for a while we relied on
> efi_ioremap.

Hmm, but you see my confusion, right? Why is init_extra_mapping_uc()
influenced by the EFI changes? It uses __init_extra_mapping() and it
maps into init_mm's pgd.

> Eventually we got a BIOS fix that allowed us to start using the new
> memmap scheme, at which point we removed the init_extra_mapping_uc()s,
> since the efi_map_region code appeared to be doing what we needed.

Why would you even do that? Why are you even mapping MMRs using EFI
facilities?

I'm more confused today.

So what I see from here is this:

* MMRs and EFI shouldn't have anything in common. Imagine there were an
UV box without EFI (you probably are going to say there's no such thing
but imagine anyway): how are you going to map the MMR space then?

* I think you should restore the old case where you mapped the MMRs
using init_extra_mapping_uc().

And I think

  d394f2d9d8e1 ("x86/platform/UV: Remove EFI memmap quirk for UV2+")

was wrong in doing

+       if (is_uv1_hub())
+               map_low_mmrs();

for the simple fact that MMRs mapping and EFI shouldn't depend on one
another.

So the proper fix would be to remove the if- check.

Or am I missing something here and MMRs need EFI?

 (However, UV1 apparently works fine without it).

Right?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--