revert scope for 5.8, was Re: dma-pool fixes

From: Christoph Hellwig <hch@lst.de>
To: David Rientjes <rientjes@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Amit Pundir <amit.pundir@linaro.org>,
	jeremy.linton@arm.com, iommu@lists.linux-foundation.org,
	linux-rpi-kernel@lists.infradead.org,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: revert scope for 5.8, was Re: dma-pool fixes
Date: Sat, 1 Aug 2020 10:57:06 +0200	[thread overview]
Message-ID: <20200801085706.GA2991@lst.de> (raw)
In-Reply-To: <alpine.DEB.2.23.453.2008010105560.4078406@chino.kir.corp.google.com>

On Sat, Aug 01, 2020 at 01:20:07AM -0700, David Rientjes wrote:
> To follow-up on this, the introduction of the DMA atomic pools in 5.8 
> fixes an issue for any AMD SEV enabled guest that has a driver that 
> requires atomic DMA allocations (for us, nvme) because runtime decryption 
> of memory allocated through the DMA API may block.  This manifests itself 
> as "sleeping in invalid context" BUGs for any confidential VM user in 
> cloud.
> 
> I unfortunately don't have Amit's device to be able to independently debug 
> this issue and certainly could not have done a better job at working the 
> bug than Nicolas and Christoph have done so far.  I'm as baffled by the 
> results as anybody else.
> 
> I fully understand the no regressions policy.  I'd also ask that we 
> consider that *all* SEV guests are currently broken if they use nvme or 
> any other driver that does atomic DMA allocations.  It's an extremely 
> serious issue for cloud.  If there is *anything* that I can do to make 
> forward progress on this issue for 5.8, including some of the workarounds 
> above that Amit requested, I'd be very happy to help.  Christoph will make 
> the right decision for DMA in 5.8, but I simply wanted to state how 
> critical working SEV guests are to users.

I'm between a rock and a hard place here.  If we simply want to revert
commits as-is to make sure both the Raspberry Pi 4 and thone phone do
not regress we'll have to go all the way back and revert the whole SEV
pool support.  I could try to manual revert of the multiple pool
support, but it is very late for that.

Or maybe Linus has decided to cut a -rc8 which would give us a little
more time.
-
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