linux-kernel - Re: [RFC/PATCH 8/9] iommu: of: Handle IOMMU lookup failure with deferred probing or error

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 28 May 2015 14:31:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Mitchel Humpherys <mitchelh@...eaurora.org>,
	Joreg Roedel <joro@...tes.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Robin Murphy <Robin.Murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC/PATCH 8/9] iommu: of: Handle IOMMU lookup failure with
 deferred probing or error

Hi Laurent,

On Fri, May 15, 2015 at 12:00:09AM +0100, Laurent Pinchart wrote:
> Failures to look up an IOMMU when parsing the DT iommus property need to
> be handled separately from the .of_xlate() failures to support deferred
> probing.
> 
> The lack of a registered IOMMU can be caused by the lack of a driver for
> the IOMMU, the IOMMU device probe not having been performed yet, having
> been deferred, or having failed.
> 
> The first case occurs when the device tree describes the bus master and
> IOMMU topology correctly but no device driver exists for the IOMMU yet
> or the device driver has not been compiled in. Return NULL, the caller
> will configure the device without an IOMMU.
> 
> The second and third cases are handled by deferring the probe of the bus
> master device which will eventually get reprobed after the IOMMU.
> 
> The last case is currently handled by deferring the probe of the bus
> master device as well. A mechanism to either configure the bus master
> device without an IOMMU or to fail the bus master device probe depending
> on whether the IOMMU is optional or mandatory would be a good
> enhancement.

I appreciate that you're just looking to handle early initialisation
failures here, but do you have any thoughts on how to deal with failures
later on when e.g. the DMA-mapping API is trying to create IOMMU domains.

One potential problem I foresee is if we try to add all devices to a common
DMA domain, we may get -ENOSPC-style failures due to limited resources on
the IOMMU. In this case, we'd probably want to fall-back to non-IOMMU DMA
ops, but that in-turn could have consequences on things like dma-coherent.

It's all a bit murky, so I'd be glad to hear any thoughts you might have
around this.

Anyway, this patch looks fine:

  Acked-by: Will Deacon <will.deacon@....com>

but we should consider how all of this will get used too.

Will
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/