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

From: Tyler Hicks <code@tyhicks.com>
To: Will Deacon <will@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Jerry Snitselaar <jsnitsel@redhat.com>,
	linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org, Dexuan Cui <decui@microsoft.com>,
	Easwar Hariharan <eahariha@linux.microsoft.com>
Subject: Re: Why is the ARM SMMU v1/v2 put into bypass mode on kexec?
Date: Tue, 19 Mar 2024 14:14:26 -0500	[thread overview]
Message-ID: <ZfnkEqglNPRzH3Zk@sequoia> (raw)
In-Reply-To: <20240319154756.GB2901@willie-the-truck>

On 2024-03-19 15:47:56, Will Deacon wrote:
> On Tue, Mar 19, 2024 at 12:57:52PM +0000, Robin Murphy wrote:
> > Beyond properly quiescing and resetting the system back to a boot-time
> > state, the outgoing kernel in a kexec can only really do things which affect
> > itself. Sure, we *could* configure the SMMU to block all traffic and disable
> > the interrupt to avoid getting stuck in a storm of faults on the way out,
> > but what does that mean for the incoming kexec payload? That it can have the
> > pleasure of discovering the SMMU, innocently enabling the interrupt and
> > getting stuck in an unexpected storm of faults. Or perhaps just resetting
> > the SMMU into a disabled state and thus still unwittingly allowing its
> > memory to be corrupted by the previous kernel not supporting kexec properly.
> 
> Right, it's hard to win if DMA-active devices weren't quiesced properly
> by the outgoing kernel. Either the SMMU was left in abort (leading to the
> problems you list above) or the SMMU is left in bypass (leading to possible
> data corruption). Which is better?

My thoughts are that a loud and obvious failure (via unidentified stream
fault messages and/or a possible interrupt storm preventing the new
kernel from booting) is favorable to silent and subtle data corruption
of the target kernel.

> The best solution is obviously to implement those missing ->shutdown()
> callbacks.

Completely agree here but it can be difficult to even identify that a
missing ->shutdown hook is the root cause without code changes to put
the SMMU into abort mode and sleep for a bit in the SMMU's ->shutdown
hook.

Tyler

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