Re: [PATCH] x86-64: disable the GART before allocate aperture

From: ebiederm@xmission.com (Eric W. Biederman)
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Muli Ben-Yehuda <muli@il.ibm.com>,
	Yinghai Lu <Yinghai.Lu@Sun.COM>, Andi Kleen <ak@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vivek Goyal <vgoyal@in.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86-64: disable the GART before allocate aperture
Date: Fri, 22 Jun 2007 16:32:21 -0600	[thread overview]
Message-ID: <m1tzszaadm.fsf@ebiederm.dsl.xmission.com> (raw)
In-Reply-To: <20070622231951.4d516215@the-village.bc.nu> (Alan Cox's message of "Fri, 22 Jun 2007 23:19:51 +0100")

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

>> YH do you think you can look at simply reserving a portion of the iommu?
>> And having the kexec on panic kernel use the reserved portion?
>
> How about simply reserving all of it for the base kernel and using soft
> iommu for the panic kernel, its hardly high performance criticial at this
> point.

The original design came from thinking about systems where using the iommu
was mandatory.  I think we almost always reserve memory below 1G for the kexec
on panic kernel so it really shouldn't be an issue in that case.  Except
we need to pass an option to force not using the iommu.  I don't think
noiommu or swiotlb is going to make any real difference.

So I'm totally in favor of turning off features if we don't need them and we
don't take a tremendous performance hit.  (People get grumpy when writing
all of memory to disk takes completely unreasonable amounts of time).

Eric