linux-kernel - Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

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Message-ID: <20180925090948.k62lms4t5qduwnpo@8bytes.org>
Date:   Tue, 25 Sep 2018 11:09:48 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     murphyt7@....ie
Cc:     leo.duran@....com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic
 context which is necessary for any custom drivers using the IOMMU API while
 holding a spinlock.

On Fri, Aug 24, 2018 at 02:28:49PM +0000, murphyt7@....ie wrote:
> From: Tom Murphy <murphyt7@....ie>
> 
> ---
> 
> This patch allows the IOMMU API path in the AMD driver to be called
> from an atomic context.  This is useful for anyone building a driver
> which needs to call the IOMMU API while holding a spinlock.

A bit more context would be helpful here. This is a pretty intrusive
change since we will drain the memory allocators atomic pools now while
building page-tables. This needs some more justification than just 'some
drivers might need it'.

Further, if we really address this problem it needs to happen through
the IOMMU-API, because the functions you are patching are called from
iommu_map()/iommu_unmap(), and these functions are allowed to sleep.

So the required approach here would be to introduce something like
iommu_map_atomic()/iommu_unmap_atomic() and implement call-backs for
that in more than just one driver.

Regards,

	Joerg