linux-kernel - Re: [PATCH] x86, kdump: Set crashkernel

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Date:	Mon, 11 Mar 2013 10:48:53 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>, WANG Chao <chaowang@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, kdump: Set crashkernel_low automatically

On Sun, Mar 10, 2013 at 09:56:57PM -0700, Yinghai Lu wrote:
> Current code does not set low range for crashkernel if the user
> does not specify that.
> 
> That cause regressions on system that does not support intel_iommu
> properly.
> 
> Chao said that his system does work well on 3.8 without extra parameter.
> even iommu does not work with kdump.
> 
> Set crashkernel_low automatically if the user does not specify that.
> 
> For system that does support IOMMU with kdump properly, user could
> specify crashkernel_low=0 to save that 72M low ram.
> 

Hi Yinghai,

Had a question about crashkernel_auto_low. So this is the amount of
memory rerved under 4G. I am not very clear about the semantics here.

So by default memory wil always come from areas above 4G (when
crashkernel=X specified) and if user needs reserveation in lower memory
area, it needs to be sepcified explicitly using crashkernel_low?

But will that not break the case of exising bzImage which are 32bit.
They currently work if I specify crashkernel=256M. Now suddenly memory
will come from higher addresses and 32bit bzImage can't be loaded there.
Or I understood the syntax part wrong.

P.S. Explanation in kernel-parameters.txt is really short. Can you please
     add some explanation in kdump.txt to clarify how crashkernel_low
    is supposed to be used.

Thanks
Vivek
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