On Fri, Mar 18, 2022 at 05:42:58PM +0000, Robin Murphy wrote: > 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. > > CC: Mario Limonciello <mario.limonciello@xxxxxxx> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx> Looks sensible to me: Acked-by: Christoph Hellwig <hch@xxxxxx>
