linux-kernel - Re: [PATCH 4/5] iommu: Separate IOVA rcache memories from iova

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Date:   Mon, 20 Dec 2021 13:57:17 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, mst@...hat.com, jasowang@...hat.com
Cc:     xieyongji@...edance.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        virtualization@...ts.linux-foundation.org, linuxarm@...wei.com,
        thunder.leizhen@...wei.com, baolu.lu@...ux.intel.com
Subject: Re: [PATCH 4/5] iommu: Separate IOVA rcache memories from iova_domain
 structure

Hi John,

On 2021-12-20 08:49, John Garry wrote:
> On 24/09/2021 11:01, John Garry wrote:
>> Only dma-iommu.c and vdpa actually use the "fast" mode of IOVA alloc and
>> free. As such, it's wasteful that all other IOVA domains hold the rcache
>> memories.
>>
>> In addition, the current IOVA domain init implementation is poor
>> (init_iova_domain()), in that errors are ignored and not passed to the
>> caller. The only errors can come from the IOVA rcache init, and fixing up
>> all the IOVA domain init callsites to handle the errors would take some
>> work.
>>
>> Separate the IOVA rache out of the IOVA domain, and create a new IOVA
>> domain structure, iova_caching_domain.
>>
>> Signed-off-by: John Garry <john.garry@...wei.com>
> 
> Hi Robin,
> 
> Do you have any thoughts on this patch? The decision is whether we stick 
> with a single iova domain structure or support this super structure for 
> iova domains which support the rcache. I did not try the former - it 
> would be do-able but I am not sure on how it would look.

TBH I feel inclined to take the simpler approach of just splitting the 
rcache array to a separate allocation, making init_iova_rcaches() public 
(with a proper return value), and tweaking put_iova_domain() to make 
rcache cleanup conditional. A residual overhead of 3 extra pointers in 
iova_domain doesn't seem like *too* much for non-DMA-API users to bear. 
Unless you want to try generalising the rcache mechanism completely away 
from IOVA API specifics, it doesn't seem like there's really enough to 
justify the bother of having its own distinct abstraction layer.

Cheers,
Robin.