linux-kernel - Re: [PATCH 00/13] Introduce IOMMU-API TLB Flushing Interface

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Date:   Thu, 17 Aug 2017 08:54:07 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Subject: Re: [PATCH 00/13] Introduce IOMMU-API TLB Flushing Interface

On Thu, 17 Aug 2017 16:43:08 +0200
Joerg Roedel <joro@...tes.org> wrote:

> Hi Alex,
> 
> On Thu, Aug 17, 2017 at 08:35:20AM -0600, Alex Williamson wrote:
> > Wouldn't it be much more friendly to downstreams and out-of-tree
> > drivers to introduce new functions for the async semantics?  ie.
> > iommu_map_async(), etc.  The API also seems a little cleaner that
> > iommu_map() stands alone, it's synchronous, iommu_map_async() is
> > explicitly asynchronous and a _flush() call is needed to finalize it.
> > What do you see as the advantage to the approach here?  Thanks,  
> 
> The reason I did it this way was that I want the iommu_map(),
> iommu_unmap(), and iomu_map_sg() functions be considered the _default_
> to chose when using the IOMMU-API, because their use is faster than
> using the _sync() variants. Or in other words, I want the _sync function
> names to imply that they are slower versions of the default ones.

So _sync() does imply that they're slower, but iommu_map() does not
imply that a _flush() is required.  One is a performance issue, the
other is an API usability issue.  If the sync version is used
sub-optimally, it's a performance issue, not a correctness issue.  If
the async version is used without an explicit flush, it's a correctness
issue.  Therefore, I would lean towards making the asynchronous mode
explicit and providing good documentation and comments to steer
developers to the async version.  I think it makes the API harder to
use incorrectly.  Thanks,

Alex