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

[Dave Young]

On 05/07/15 at 10:25am, Don Dutile wrote:
> 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.

So NMI_IOCK is one reason to cause kernel hang, I think I'm still not clear about
what does re-configured means though. DMAR faults will happen originally this is the old
behavior but we are removing the faults by alowing DMA continuing into old 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?

Just do not know why the previous handling is necessary with iommu=off, shouldn't
we do noting and return earlier?

Also there is a guess, dmar faults appears after iommu_init, so not sure if the codes
before dmar_disabled checking have some effect about enabling the faults messages.

Thanks
Dave
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