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

From: Borislav Petkov
Date: Wed Jun 21 2017 - 12:59:46 EST

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.




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