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

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Date:	Sun, 07 Oct 2012 09:16:49 +0100
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<hpa@...or.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

>>> "H. Peter Anvin" <hpa@...or.com> 10/05/12 6:28 PM >>>
>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. :)

I understand all that, but here we're talking about a true 1:1 mapping
(i.e. in the lower half of the virtual address space), as opposed to the
direct mapping (in the upper half). We can obviously use all of the lower
half for this purpose.

Jan

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