linux-kernel - Re: [PATCH v2] Do not reserve crashkernel high memory if crashkernel low memory reserving failed

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Message-ID: <20150728005242.GB1720@dhcp-17-102.nay.redhat.com>
Date:	Tue, 28 Jul 2015 08:52:42 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Dave Young <dyoung@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Vivek Goyal <vgoyal@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Do not reserve crashkernel high memory if crashkernel
 low memory reserving failed

On 07/22/15 at 04:47pm, Yinghai Lu wrote:
> On Tue, Jul 21, 2015 at 5:59 PM, Baoquan He <bhe@...hat.com> wrote:
> >> That commit should only be used to workaround some systems that
> >> have partial iommu support.
> >
> > Those big servers mostly has hardware iommu. But they still can
> > enable swiotlb suport. Then low memory is needed.
> 
> Do you have whole bootlog? I don't understand why those system can not use
> full iommu. BIOS problem or HW/silicon limitation?

Sorry for late reply. This problem is reported by customers. They usulay
don't like to make these things public. While for those systems with
good hard iommu support, it could also fail to initialize hw iommu and
then use swiotlb again, E.g in kdump kernel case. You can see in
intel_iommu_init() swiotlb is assigned to 0 only if intel iommu (namely vt-d)
is initialized successfully. There's possibility that hw iommu
initialization will fail, in this case kdump kernel will fail to boot
if no any low memory is given. So we can't make assumption that system
can boot always well without low memory.

Thanks
Baoquan
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