Re: [PATCH v3 2/5] iommu/amd: Fix DTE clearing and rename iommu_ignore_device()

[Jason Gunthorpe]

On Fri, Sep 04, 2026 at 03:43:01PM +0530, Vasant Hegde wrote:
> Jason,
> 
> 
> On 8/28/2026 5:23 PM, Jason Gunthorpe wrote:
> > On Fri, Aug 28, 2026 at 10:46:48AM +0530, Vasant Hegde wrote:
> >>> Having the driver boot up with all DTEs programmed to identity (eg
> >>> 0'd) and then try to fix them to blocking after the iommu probes
> >>> devices is security backwards.
> >>
> >> During boot, it only sets dte.v bit.
> > 
> > First it clears it to fully 0, what does 0 do in HW?
> 
> IF DTE is fully zero, then all requests are blocked for that devid.
> 
> > 
> > It doesn't make sense that you'd pass over the DTEs after
> > probing if the original 0'd DTE was actually blocking?
> 
> During boot, it sets certain default values includ dte.v. It doesn't
> clear everything.

?? It starts out with a 0 DTE table? There is no inherited DTE table
except for kdump.

> May be we should just remove ignore_device() completely? as
>   - normal boot, its not yet configured, so no DMA is allowed
>   - kdump boot, old DTE is still valid and let it continue?

Yes, that makes alot more sense to me.

But this comment is also wrong:

        /*
         * Order is important here to make sure any unity map requirements are
         * fulfilled. The unity mappings are created and written to the device
         * table during the iommu_init_pci() call.
         *
         * After that we call init_device_table_dma() to make sure any
         * uninitialized DTE will block DMA, and in the end we flush the caches
         * of all IOMMUs to make sure the changes to the device table are
         * active.
         */
        for_each_pci_segment(pci_seg)
                init_device_table_dma(pci_seg);

The DTE starts out with blocking because it starts out as 0. This
isn't making the DTE blocking, it is doing something else. And it is
very suspicious and racey looking to me.

The DTE should start out in the right state before the DTE table is
programmed into HW. After that point the only places changing a DTE
should be inside a device attach attach callback.

Probe and this global scan should not touch the DTE.

Jason