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: Jason Gunthorpe <jgg@ziepe.ca>,
	Robin Murphy <robin.murphy@arm.com>,
	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: Fri, 22 Mar 2024 14:52:52 -0500	[thread overview]
Message-ID: <Zf3hlOcYd7LB6Xvj@sequoia> (raw)
In-Reply-To: <20240322155529.GE5634@willie-the-truck>

On 2024-03-22 15:55:29, Will Deacon wrote:
> Hey Jason,
> 
> On Tue, Mar 19, 2024 at 02:50:07PM -0300, Jason Gunthorpe wrote:
> > On Tue, Mar 19, 2024 at 03:47:56PM +0000, Will Deacon wrote:
> > 
> > > 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?
> > 
> > For whatever reason (and I really don't like this design) alot of work
> > was done on x86 so that device continues to work as-was right up until
> > the crash kernel does the first DMA operation. Including having the
> > crash kernel non disruptively inherit and retain the IOMMU
> > configuration. (eg see translation_pre_enabled() stuff in intel
> > driver)
> 
> Right, I'm also not thrilled about trying to implement that :)
> What we have at the moment seems to be good enough to avoid folks
> complaining about it.
> 
> For the case Tyler is reporting, though, I _think_ it's just a standard
> kexec() rather than a crashkernel.

That's correct.

Tyler