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Date: Wed, 21 Jun 2017 18:59:22 +0200
From: Borislav Petkov <bp@alien8.de>
To: Joerg Roedel <joro@8bytes.org>
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Subject: Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with
 memory encryption
Message-ID: <20170621165921.tv2jfhf5dz7hsjsy@pd.tnic>
References: <20170607191309.28645.15241.stgit@tlendack-t1.amdoffice.net>
 <20170607191745.28645.81756.stgit@tlendack-t1.amdoffice.net>
 <20170614174208.p2yr5exs4b6pjxhf@pd.tnic>
 <0611d01a-19f8-d6ae-2682-932789855518@amd.com>
 <20170615094111.wga334kg2bhxqib3@pd.tnic>
 <20170621153721.GP30388@8bytes.org>
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On Wed, Jun 21, 2017 at 05:37:22PM +0200, Joerg Roedel wrote:
> > Do you mean this is like the last exception case in that document above:
> > 
> > "
> >   - Pointers to data structures in coherent memory which might be modified
> >     by I/O devices can, sometimes, legitimately be volatile.  A ring buffer
> >     used by a network adapter, where that adapter changes pointers to
> >     indicate which descriptors have been processed, is an example of this
> >     type of situation."
> > 
> > ?
> 
> So currently (without this patch) the build_completion_wait function
> does not take a volatile parameter, only wait_on_sem() does.
> 
> Wait_on_sem() needs it because its purpose is to poll a memory location
> which is changed by the iommu-hardware when its done with command
> processing.

Right, the reason above - memory modifiable by an IO device. You could
add a comment there explaining the need for the volatile.

> But the 'volatile' in build_completion_wait() looks unnecessary, because
> the function does not poll the memory location. It only uses the
> pointer, converts it to a physical address and writes it to the command
> to be queued.

Ok.

Thanks.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.