linux-kernel - Re: [PATCH v2 5/6] powerpc/pseries/iommu: Make use of DDW even if it does not map the partition

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Message-ID: <14da928d1cae8779a93228455c4873c127eeffa6.camel@gmail.com>
Date:   Fri, 26 Jun 2020 12:23:28 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] powerpc/pseries/iommu: Make use of DDW even if
 it does not map the partition

On Wed, 2020-06-24 at 03:24 -0300, Leonardo Bras wrote:
> As of today, if a DDW is created and can't map the whole partition, it's
> removed and the default DMA window "ibm,dma-window" is used instead.
> 
> Usually this DDW is bigger than the default DMA window, so it would be
> better to make use of it instead.
> 
> Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---

I tested this change with a 256GB DDW which did not map the whole
partition, with a MT27700 Family [ConnectX-4 Virtual Function].

I noticed the performance improvement is about the same as using DDW
with IOMMU bypass.

64 thread write throughput: +203.0%
64 thread read throughput: +17.5%
1 thread write throughput: +20.5%
1 thread read throughput: +3.43%
Averag
e write latency: -23.0%
Average read latency:  -2.26%