Messages in this thread | | | Date | Mon, 21 Mar 2022 11:11:42 +0000 | Subject | Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate | From | Robin Murphy <> |
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On 2022-03-21 10:58, mika.westerberg@linux.intel.com wrote: > 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.
And even if there are, as above they've never actually been fully protected and still won't be, so it's arguably a good thing for them to stop thinking so.
>> 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 :)
Heh, let's just hope the other half-dozen or so subsystems I need to touch for this IOMMU cleanup aren't all quite as involved as this turned out to be :)
Cheers, Robin.
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