linux-kernel - Re: [PATCH v9 0/10] iommu/vt-d: Fix intel vt-d faults in kdump kernel

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Message-ID: <20150407034622.GB7213@localhost.localdomain>
Date:	Tue, 7 Apr 2015 11:46:22 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Baoquan He <bhe@...hat.com>
Cc:	"Li, ZhenHua" <zhen-hual@...com>, dwmw2@...radead.org,
	indou.takao@...fujitsu.com, joro@...tes.org, vgoyal@...hat.com,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, kexec@...ts.infradead.org,
	alex.williamson@...hat.com, ddutile@...hat.com,
	ishii.hironobu@...fujitsu.com, bhelgaas@...gle.com,
	doug.hatch@...com, jerry.hoemann@...com, tom.vaden@...com,
	li.zhang6@...com, lisa.mitchell@...com, billsumnerlinux@...il.com,
	rwright@...com
Subject: Re: [PATCH v9 0/10] iommu/vt-d: Fix intel vt-d faults in kdump kernel

On 04/05/15 at 09:54am, Baoquan He wrote:
> On 04/03/15 at 05:21pm, Dave Young wrote:
> > On 04/03/15 at 05:01pm, Li, ZhenHua wrote:
> > > Hi Dave,
> > > 
> > > There may be some possibilities that the old iommu data is corrupted by
> > > some other modules. Currently we do not have a better solution for the
> > > dmar faults.
> > > 
> > > But I think when this happens, we need to fix the module that corrupted
> > > the old iommu data. I once met a similar problem in normal kernel, the
> > > queue used by the qi_* functions was written again by another module.
> > > The fix was in that module, not in iommu module.
> > 
> > It is too late, there will be no chance to save vmcore then.
> > 
> > Also if it is possible to continue corrupt other area of oldmem because
> > of using old iommu tables then it will cause more problems.
> > 
> > So I think the tables at least need some verifycation before being used.
> > 
> 
> Yes, it's a good thinking anout this and verification is also an
> interesting idea. kexec/kdump do a sha256 calculation on loaded kernel
> and then verify this again when panic happens in purgatory. This checks
> whether any code stomps into region reserved for kexec/kernel and corrupt
> the loaded kernel.
> 
> If this is decided to do it should be an enhancement to current
> patchset but not a approach change. Since this patchset is going very
> close to point as maintainers expected maybe this can be merged firstly,
> then think about enhancement. After all without this patchset vt-d often
> raised error message, hung.

It does not convince me, we should do it right at the beginning instead of
introduce something wrong.

I wonder why the old dma can not be remap to a specific page in kdump kernel
so that it will not corrupt more memory. But I may missed something, I will
looking for old threads and catch up.

Thanks
Dave
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