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

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Date:	Tue, 7 Apr 2015 17:08:37 +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 04/07/15 at 11:46am, Dave Young wrote:
> 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.

I have read the old discussion, above way was dropped because it could corrupt
filesystem. Apologize about late commenting.

But current solution sounds bad to me because of using old memory which is not
reliable. 

Thanks
Dave
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