linux-kernel - Re: [PATCH] iommu/iova: Update cached node pointer when current node fails to get any free IOVA

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Date:   Fri, 27 Jul 2018 17:18:54 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Ganapatrao Kulkarni <gklkml16@...il.com>
Cc:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>, tomasz.nowicki@...ium.com,
        jnair@...iumnetworks.com,
        Robert Richter <Robert.Richter@...ium.com>,
        Vadim.Lomovtsev@...ium.com, Jan.Glauber@...ium.com
Subject: Re: [PATCH] iommu/iova: Update cached node pointer when current node
 fails to get any free IOVA

On 27/07/18 13:56, Ganapatrao Kulkarni wrote:
[...]
>>> did you get any chance to look in to this issue?
>>> i am waiting for your suggestion/patch for this issue!
>>
>>
>> I got as far as [1], but I wasn't sure how much I liked it, since it still
>> seems a little invasive for such a specific case (plus I can't remember if
>> it's actually been debugged or not). I think in the end I started wondering
>> whether it's even worth bothering with the 32-bit optimisation for PCIe
>> devices - 4 extra bytes worth of TLP is surely a lot less significant than
>> every transaction taking up to 50% more bus cycles was for legacy PCI.
> 
> how about tracking previous attempt to get 32bit range iova and avoid
> further attempts, if it was failed. Later Resume attempts once
> replenish happens.
> Created patch for the same [2]

Ooh, that's a much neater implementation of essentially the same concept 
- now why couldn't I think of that? :)

Looks like it should be possible to make it entirely self-contained too, 
since alloc_iova() is in a position to both test and update the flag 
based on the limit_pfn passed in.

Robin.

> 
> [2] https://github.com/gpkulkarni/linux/commit/e2343a3e1f55cdeb5694103dd354bcb881dc65c3
> note, the testing of this patch is in progress.
> 
>>
>> Robin.
>>
>> [1]
>> http://www.linux-arm.org/git?p=linux-rm.git;a=commitdiff;h=a8e0e4af10ebebb3669750e05bf0028e5bd6afe8
> 
> thanks
> Ganapat
>