[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.
>