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

[Tom Lendacky]

On 6/21/2017 11:59 AM, Borislav Petkov wrote:
> 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.

Ok, so the (now) current version of the patch that doesn't change the
function signature is the right way to go.

Thanks,
Tom

> Thanks.