linux-kernel - Re: Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support"

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Message-ID: <1424409761.27448.32.camel@kernel.crashing.org>
Date:	Fri, 20 Feb 2015 16:22:41 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Sreekanth Reddy <Sreekanth.Reddy@...gotech.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	scsi <linux-scsi@...r.kernel.org>, Christoph Hellwig <hch@....de>
Subject: Re: Concerns about "mpt2sas: Added Reply Descriptor Post Queue
 (RDPQ) Array support"

On Fri, 2015-02-20 at 16:06 +1100, Benjamin Herrenschmidt wrote:

> Note that even on powerpc platforms where it would work because we
> maintain both 32-bit and 64-bit bypass windows in the device address
> space simultaneously, you will leak iommu entries unless you also switch
> back to 32-bit when freeing the 32-bit mappings... (and you would
> probably crash if you tried to free a 64-bit mapping while in 32-bit
> mode).
> 
> The iommu APIs weren't designed with that "switching mask" facility in
> mind...

Looking a bit more closely, you basically do

 - set_dma_mask(64-bit)
 - set_consistent_dma_mask(32-bit)

Now, I don't know how x86 will react to the conflicting masks, but on
ppc64, I'm pretty sure the second one will barf. IE, the first one will
establish a set of direct mapping ops which give you a bypass of the
iommu to all of memory. The second one will then do a
dma_supported(mask) call which will hit the direct ops, and they will
fail since a 32-bit mask cannot address the bypass completely.

Are architectures really required to support such mismatching dma_mask
and consistent_dma_mask ? what a bloody trainwreck ... :-(

Cheers,
Ben.


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