Re: [PATCH] iommu/amd - Optimize PPR log handling

[Gary R Hook]

On 05/29/2018 09:54 AM, Joerg Roedel wrote:
> Hey Gary,
>
> On Fri, May 18, 2018 at 04:51:56PM -0500, Gary R Hook wrote:
> > Improve the performance of the PPR log polling function (i.e. the
> > task of emptying the log) by minimizing MMIO operations and more
> > efficiently processing groups of log entries. Cache the head
> > pointer, as there's never a reason to read it. Ensure the head
> > pointer register is updated every so often, to inform the IOMMU
> > that space is available in the log.
> >
> > Finally, since a single pass may leave logged events unread, use
> > an outer loop to repeat until head has caught up to tail.
> >
> > Signed-off-by: Gary R Hook <gary.hook@xxxxxxx>
>
> Do you have numbers for the performance improvement? How did you test
> this patch?

No, no numbers. We're still working out how best to test this, and 
suggestions/strategies are welcome.

The change is modeled after the function iommu_poll_events(), which is 
much cleaner. The GA log handling should be changed, as well (there are 
superfluous writes in the loop), but I figured, "one thing at a time". 
This is admittedly a minor optimization, but discussions with Tom 
Lendacky have led us down this path.

Your feedback is appreciated.

Gary