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

[Don Dutile]

On 05/07/2015 10:00 AM, Dave Young wrote:
> On 04/07/15 at 10:12am, Don Dutile wrote:
> > On 04/06/2015 11:46 PM, 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.
> > >
> > > Thanks
> > > Dave
> > The (only) issue is not corruption, but once the iommu is re-configured, the old,
> > not-stopped-yet, dma engines will use iova's that will generate dmar faults, which
> > will be enabled when the iommu is re-configured (even to a single/simple paging scheme)
> > in the kexec kernel.
>
> Don, so if iommu is not reconfigured then these faults will not happen?
Well, if iommu is not reconfigured, then if the crash isn't caused by
an IOMMU fault (some systems have firmware-first catch the IOMMU fault & convert
them into NMI_IOCK), then the DMA's will continue into the old kernel memory space.

> Baoquan and me has a confusion below today about iommu=off/intel_iommu=off:
>
> intel_iommu_init()
> {
> ...
>
>         dmar_table_init();
>
>         disable active iommu translations;
>
>         if (no_iommu || dmar_disabled)
>                  goto out_free_dmar;
>
> ...
> }
>
> Any reason not move no_iommu check to the begining of intel_iommu_init function?
What does that do/help?

> Thanks
> Dave

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