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

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Date:   Thu, 28 Apr 2022 23:54:31 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Robin Murphy <robin.murphy@....com>, hch@...radead.org,
        m.szyprowski@...sung.com, michael.h.kelley@...rosoft.com,
        kys@...rosoft.com
Cc:     parri.andrea@...il.com, thomas.lendacky@....com,
        wei.liu@...nel.org, Andi Kleen <ak@...ux.intel.com>,
        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 4/28/2022 10:44 PM, Robin Murphy wrote:
> On 2022-04-28 15:14, Tianyu Lan wrote:
>> From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
>>
>> Traditionally swiotlb was not performance critical because it was only
>> used for slow devices. But in some setups, like TDX/SEV confidential
>> guests, all IO has to go through swiotlb. Currently swiotlb only has a
>> single lock. Under high IO load with multiple CPUs this can lead to
>> significat lock contention on the swiotlb lock.
>>
>> This patch splits the swiotlb into individual areas which have their
>> own lock. When there are swiotlb map/allocate request, allocate
>> io tlb buffer from areas averagely and free the allocation back
>> to the associated area. This is to prepare to resolve the overhead
>> of single spinlock among device's queues. Per device may have its
>> own io tlb mem and bounce buffer pool.
>>
>> This idea from Andi Kleen 
>> patch(https://github.com/intel/tdx/commit/4529b578
>> 4c141782c72ec9bd9a92df2b68cb7d45). Rework it and make it may work
>> for individual device's io tlb mem. The device driver may determine
>> area number according to device queue number.
> 
> Rather than introduce this extra level of allocator complexity, how 
> about just dividing up the initial SWIOTLB allocation into multiple 
> io_tlb_mem instances?
> 
> Robin.

Agree. Thanks for suggestion. That will be more generic and will update
in the next version.

Thanks.