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

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Date:	Fri, 29 Apr 2016 10:01:15 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org, 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, 27 Apr, at 10:41:32AM, Alex Thorlton wrote:
> 
> From what I know, this works because the first PGD entry (the one
> containing the identity mappings) of the trampoline_pgd is shared with
> the main kernel PGD (init_level4_pgt), so when __map_region maps this
> stuff into the trampoline_pgd, we get it in the kernel PGD as well.

Correct.

> The way I understand it, the motivation behind this change is to isolate
> the EFI runtime services code/data from the regular kernel page tables,

Yep.

> but it appears that we've isolated a bit more than that.  I do
> understand that if we were doing an actual EFI callback, we'd switch
> over to the efi_pgd, where we'd have this stuff mapped in, but, until
> now, we've been able to read these MMRs using the regular kernel page
> table without issues.
> 
> Please let me know what you guys think about this issue, and if you have
> any suggestions for possible solutions.  Any input is greatly
> appreciated!

The issue is that you're relying on the EFI mapping code to map these
regions for you, and that's a bug. These regions have nothing to do
with EFI.

Arguably it's always been a bug. You're not alone though, there were
also other pieces of code piggy-backing on the EFI mapping functions,
and EFI code piggy-backing on the regular kernel mapping functions,
see 452308de6105 ("x86/efi: Fix boot crash by always mapping boot
service regions into new EFI page tables").

I agree with Boris' suggestion that removing the "if (is_uv1_hub())"
check in uv_system_init() is probably the way to go, since it should
always be the responsibility of uv_system_init() to setup these
mappings.