linux-kernel - Re: [PATCH] x86_64/efi: Mapping Boot and Runtime EFI memory regions to different starting virtual address

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Date:	Thu, 30 Jul 2015 15:05:42 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	joeyli <jlee@...e.com>
Cc:	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64/efi: Mapping Boot and Runtime EFI memory regions
 to different starting virtual address

On Thu, 30 Jul, at 09:39:47PM, joeyli wrote:
> 
> OK, understood! Thanks for your suggestion!
> 
> But, I have a question about mapping Boot Code/Data to -4G area. I understand
> we need Runtime regions, and 1:1 mapping is the workaround of some buggy BIOS.
> But why should kernel mapping Boot regions to -4G area?

We map the Boot Service regions in the -4G area for simplicity, i.e.
it's easier to iterate through the EFI memory map in order, and map
things starting at -4G and working down to lower addresses, than it
would be to make different mapping decisions based on the region type.

As for why we map the Boot Service regions *at all*, that's also a bug
workaround, see commit 916f676f8dc0 ("x86, efi: Retain boot service code
until after switching to virtual mode"). We setup the mappings before we
call efi_free_boot_services().

-- 
Matt Fleming, Intel Open Source Technology Center
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