Re: [PATCH] thunderbolt: Make iommu_dma_protection more accurate

[mika.westerberg@xxxxxxxxxxxxxxx]

Hi Robin,

On Fri, Mar 18, 2022 at 03:15:19PM +0000, Robin Murphy wrote:
> > IMHO we should just trust the firmare provided information here
> > (otherwise we are screwed anyway as there is no way to tell if the
> > devices connected prior the OS can still do DMA), and use the external
> > facing port indicator to idenfity the ports that need DMA protection.
> 
> Indeed that's exactly what I want to do, but it begs the question of how we
> *find* the firmware-provided information in the first place!

Oh, right :) Its the combination of ACPI _DSD "ExternalFacingPort"
(which we already set, dev->external_facing, dev->untrusted for the
devices behind these ports IIRC) and the DMAR opt-in bit. All these are
already read by the kernel.

> I seem to have already started writing the dumb version that will walk the
> whole PCI segment and assume the presence of any external-facing port
> implies that we're good. Let me know if I should stop ;)

That sounds good to me, so don't stop just yet ;-)