Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with memory encryption

From	Joerg Roedel <joro@8bytes.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with memory encryption
Date	2017-06-21 18:40 +0200
Message-ID	<tUQPD-6J-1@gated-at.bofh.it> (permalink)
References	<tPRjk-44i-19@gated-at.bofh.it> <tPVwC-6GZ-15@gated-at.bofh.it> <tSkAx-8cx-7@gated-at.bofh.it> <tSzzA-MV-21@gated-at.bofh.it> <tSzzA-MV-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

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

Back to linux.kernel | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with  memory encryption Joerg Roedel <joro@8bytes.org> - 2017-06-21 18:40 +0200
  Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with  memory encryption Borislav Petkov <bp@alien8.de> - 2017-06-21 19:10 +0200

csiph-web