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

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Date:   Tue, 29 May 2018 12:28:54 -0500
From:   Gary R Hook <ghook@....com>
To:     Joerg Roedel <joro@...tes.org>, Gary R Hook <gary.hook@....com>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/amd - Optimize PPR log handling

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@....com>
> 
> 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