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

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Message-ID: <ZfKsAIt8RY/JcL/V@sequoia>
Date: Thu, 14 Mar 2024 02:49:20 -0500
From: Tyler Hicks <code@...icks.com>
To: Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Jerry Snitselaar <jsnitsel@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
	Easwar Hariharan <eahariha@...ux.microsoft.com>
Subject: Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

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!

Tyler