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@xxxxxxx> > > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx> > > --- > > > > 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 :)
