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Date: Wed, 21 Jun 2017 17:37:22 +0200
From: Joerg Roedel <joro@8bytes.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>, linux-arch@vger.kernel.org,
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Subject: Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with
 memory encryption
Message-ID: <20170621153721.GP30388@8bytes.org>
References: <20170607191309.28645.15241.stgit@tlendack-t1.amdoffice.net>
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 <20170614174208.p2yr5exs4b6pjxhf@pd.tnic>
 <0611d01a-19f8-d6ae-2682-932789855518@amd.com>
 <20170615094111.wga334kg2bhxqib3@pd.tnic>
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On Thu, Jun 15, 2017 at 11:41:12AM +0200, Borislav Petkov wrote:
> On Wed, Jun 14, 2017 at 03:40:28PM -0500, Tom Lendacky wrote:
> > > WARNING: Use of volatile is usually wrong: see Documentation/process/volatile-considered-harmful.rst
> > > #134: FILE: drivers/iommu/amd_iommu.c:866:
> > > +static void build_completion_wait(struct iommu_cmd *cmd, volatile u64 *sem)
> > > 
> > 
> > The semaphore area is written to by the device so the use of volatile is
> > appropriate in this case.
> 
> 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.

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.


Regards,

	Joerg