Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate

["mika.westerberg@linux.intel.com" <mika.westerberg@linux.intel.com>]

Hi Mario,

On Fri, Mar 18, 2022 at 10:29:59PM +0000, Limonciello, Mario wrote:
> [Public]
> 
> > Between me trying to get rid of iommu_present() and Mario wanting to
> > support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has
> > shown
> > that the iommu_dma_protection attribute is being far too optimistic.
> > Even if an IOMMU might be present for some PCI segment in the system,
> > that doesn't necessarily mean it provides translation for the device(s)
> > we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
> > is tell us that memory was protected before the kernel was loaded, and
> > prevent the user from disabling the intel-iommu driver entirely. While
> > that lets us assume kernel integrity, what matters for actual runtime
> > DMA protection is whether we trust individual devices, based on the
> > "external facing" property that we expect firmware to describe for
> > Thunderbolt ports.
> > 
> > It's proven challenging to determine the appropriate ports accurately
> > given the variety of possible topologies, so while still not getting a
> > perfect answer, by putting enough faith in firmware we can at least get
> > a good bit closer. If we can see that any device near a Thunderbolt NHI
> > has all the requisites for Kernel DMA Protection, chances are that it
> > *is* a relevant port, but moreover that implies that firmware is playing
> > the game overall, so we'll use that to assume that all Thunderbolt ports
> > should be correctly marked and thus will end up fully protected.
> > 
> 
> This approach looks generally good to me.  I do worry a little bit about older
> systems that didn't set ExternalFacingPort in the FW but were previously setting
> iommu_dma_protection, but I think that those could be treated on a quirk
> basis to set PCI IDs for those root ports as external facing if/when they come
> up.

There are no such systems out there AFAICT.

> I'll send up a follow up patch that adds the AMD ACPI table check.
> If it looks good can roll it into your series for v3, or if this series goes
> as is for v2 it can come on its own.
> 
> > CC: Mario Limonciello <mario.limonciello@amd.com>
> > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > ---
> > 
> > v2: Give up trying to look for specific devices, just look for evidence
> >     that firmware cares at all.
> 
> I still do think you could know exactly which devices to use if you're in
> SW CM mode, but I guess the consensus is to not bifurcate the way this
> can be checked.

Indeed.

The patch looks good to me now. I will give it a try on a couple of
systems later today or tomorrow and let you guys know how it went. I
don't expect any problems but let's see.

Thanks a lot Robin for working on this :)