linux-kernel - Re: [PATCH 04/13] x86, mm: Revert back good

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Message-ID: <506F7CF5.2070904@zytor.com>
Date:	Fri, 05 Oct 2012 17:36:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jacob Shin <jacob.shin@....com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/13] x86, mm: Revert back good_end setting for 64bit

On 10/05/2012 05:28 PM, Eric W. Biederman wrote:
>>
>> Seriously, any case where we can't load anywhere in physical ram on x86-64 is a
>> bug.  i386 is another matter.
>
> As I recall there are data structures like the IDT that only have a
> 32bit base address.
>

Not true.  The only one I know of is memory for the trampoline which has 
to be below 1M.  The < 1M space is already handled specially for good 
reason.

> According to the bzImage header we don't support ramdisks above 4G.
> I think we also have a 32bit address for the kernel command line
> in the bzImage header.

There are pointers in the bzImage header, that is true.  We can fix that 
problem, though, at least for entry via the 64-bit entry point.

> In the case of kdump in particular there is a need for DMAable
> memory and in general that means memory below 4G.  So as long
> as we only support one memory extent for kdump it makes sense
> for that segment to be below 4G.

"In general" meaning "no iotlb"?  In that case you have some unknown 
address space restriction which may or may not be 4G...

> For a normal x86_64 kernel which gets to use most of the memory it
> definitely should be loadable anywhere in memory.

Yes.  We should fix problems, like the limitations in the boot_params 
structure.

	-hpa

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

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