linux-kernel - Re: [PATCH 1/3] x86, mm: Include the entire kernel memory map in trampoline

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Date:	Fri, 05 Oct 2012 09:28:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	matt@...sole-pimps.org, matt.fleming@...el.com, mingo@...nel.org,
	x86@...nel.org, mjg@...hat.com, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86, mm: Include the entire kernel memory map in
 trampoline_pgd

On 10/04/2012 11:39 PM, Jan Beulich wrote:
>>
>> We should have the check, but at least for Linux support we require
>> P <= V-2.
>
> Not really imo - P <= V - 1 should be sufficient here, as all that is
> necessary is that the result represents a 1:1 mapping. Specifically,
> there's no constraint to the virtual space limitation of the direct
> mapping of RAM.
>

The P <= V-2 limitation doesn't come from this, it comes from the fact 
that we need to have the regular kernel 1:1 map and still have space for 
the kernel, vmalloc, ioremap and so on in the kernel part of the address 
space; in theory it *could* be some fraction between 1 and 2, but since 
hardware doesn't do fractional bits very well the above is what we have 
been telling the hardware folks. :)

	-hpa
-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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