Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

[Tyler Hicks]

On 2024-03-14 09:55:46, Tyler Hicks wrote:
> Given that drivers are only optionally asked to implement the .shutdown
> hook, which is required to properly quiesce devices before a kexec, why
> is it that we put the ARM SMMU v1/v2 into bypass mode in the arm-smmu
> driver's own .shutdown hook?
> 
>  arm_smmu_device_shutdown() -> set SMMU_sCR0.CLIENTPD bit to 1
> 
> Driver authors often forget to even implement a .shutdown hook, which
> results in some hard-to-debug memory corruption issues in the kexec'ed
> target kernel due to pending DMA operations happening on untranslated
> addresses. Why not leave the SMMU in translate mode but clear the stream
> mapping table (or maybe even call arm_smmu_device_reset()) in the SMMU's
> .shutdown hook to prevent the memory corruption from happening in the
> first place?
> 
> Fully acknowledging that the proper fix is to quiesce the devices, I
> feel like resetting the SMMU and leaving it in translate mode across
> kexec would be more consistent with the intent behind v5.2 commit
> 954a03be033c ("iommu/arm-smmu: Break insecure users by disabling bypass
> by default"). The incoming transactions of devices, that weren't
> properly quiesced during a kexec, would be blocked until their drivers
> have a chance to reinitialize the devices in the new kernel.
> 
> I appreciate any help understanding why bypass mode is utilized here as
> I'm sure there are nuances that I haven't considered. Thank you!

I now see that Will has previously mentioned that he'd be open to such a
change:

 One thing I would be in favour of is changing the ->shutdown() code to
 honour disable_bypass=1 so that we put the SMMU in an aborting state
 instead of passthrough. Would that help at all? It would at least
 avoid the memory corruption on missing shutdown callback.

 - https://lore.kernel.org/linux-arm-kernel/20200608113852.GA3108@willie-the-truck/

Robin mentions the need to support kexec into a non-SMMU-aware OS. I
hadn't considered that bit of complexity:

 ... consider if the first kernel *did* leave it enabled with whatever
 left-over translations in place, and kexec'ed into another OS that
 wasn't SMMU-aware...

 - https://lore.kernel.org/linux-arm-kernel/e072f61a-d6cf-2e34-efd5-c1db38c5c622@arm.com/

Now that we're 3-4 years removed from that conversation, has anything
changed? Will, is there anything we'd need to watch out for if we were
to prototype this sort of change? For example, would it be wise to
disable fault interrupts when putting the SMMU in an aborting state
before kexec'ing?

Thanks again!

Tyler