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

[Robin Murphy <robin.murphy@arm.com>]

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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