linux-kernel - Re: [RFC PATCH 1/2] swiotlb: Split up single swiotlb lock

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Message-ID: <aa8e2fab-5b7e-cac3-0fbd-7c6edbbf942a@arm.com>
Date:   Thu, 28 Apr 2022 16:07:49 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Andi Kleen <ak@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Tianyu Lan <ltykernel@...il.com>, m.szyprowski@...sung.com,
        michael.h.kelley@...rosoft.com, kys@...rosoft.com,
        parri.andrea@...il.com, thomas.lendacky@....com,
        wei.liu@...nel.org, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-hyperv@...r.kernel.org, konrad.wilk@...cle.com,
        linux-kernel@...r.kernel.org, kirill.shutemov@...el.com,
        iommu@...ts.linux-foundation.org, andi.kleen@...el.com,
        brijesh.singh@....com, vkuznets@...hat.com, hch@....de
Subject: Re: [RFC PATCH 1/2] swiotlb: Split up single swiotlb lock

On 2022-04-28 15:55, Andi Kleen wrote:
> 
> On 4/28/2022 7:45 AM, Christoph Hellwig wrote:
>> On Thu, Apr 28, 2022 at 03:44:36PM +0100, Robin Murphy wrote:
>>> Rather than introduce this extra level of allocator complexity, how 
>>> about
>>> just dividing up the initial SWIOTLB allocation into multiple io_tlb_mem
>>> instances?
>> Yeah.  We're almost done removing all knowledge of swiotlb from drivers,
>> so the very last thing I want is an interface that allows a driver to
>> allocate a per-device buffer.
> 
> At least for TDX need parallelism with a single device for performance.
> 
> So if you split up the io tlb mems for a device then you would need a 
> new mechanism to load balance the requests for single device over those. 
> I doubt it would be any simpler.

Eh, I think it would be, since the round-robin retry loop can then just 
sit around the existing io_tlb_mem-based allocator, vs. the churn of 
inserting it in the middle, plus it's then really easy to statically 
distribute different starting points across different devices via 
dev->dma_io_tlb_mem if we wanted to.

Admittedly the overall patch probably ends up about the same size, since 
it likely pushes a bit more complexity into swiotlb_init to compensate, 
but that's still a trade-off I like.

Thanks,
Robin.