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Subject: Re: [PATCH v9 6/6] iommu/dma: Make flush queue sizes and timeout
 driver configurable
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Matthew Rosato <mjrosato@linux.ibm.com>,
        Will Deacon <will@kernel.org>,
        Wenjia Zhang <wenjia@linux.ibm.com>,
        Robin Murphy <robin.murphy@arm.com>,
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        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
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        Jonathan Hunter <jonathanh@nvidia.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-s390@vger.kernel.org,
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Date:   Tue, 23 May 2023 14:20:24 +0200
In-Reply-To: <ZGuT2R42SWFHmklu@8bytes.org>
References: <20230310-dma_iommu-v9-0-65bb8edd2beb@linux.ibm.com>
         <20230310-dma_iommu-v9-6-65bb8edd2beb@linux.ibm.com>
         <ZGuT2R42SWFHmklu@8bytes.org>
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On Mon, 2023-05-22 at 18:10 +0200, Joerg Roedel wrote:
> On Mon, May 15, 2023 at 11:15:56AM +0200, Niklas Schnelle wrote:
> > In the s390 IOMMU driver a large fixed queue size and timeout is then
> > set together with single queue mode bringing its performance on s390
> > paged memory guests on par with the previous s390 specific DMA API
> > implementation.
>=20
> Hmm, the right flush-queue size and timeout settings are more a function
> of the endpoint device and device driver than of the iommu driver, no? I
> think something like this could also help solving the recently reported
> scalability problems in the fq-code, if done right.
>=20
> Regards,
>=20
> 	Joerg
>=20

In our case the large flush queue and timeout is needed because the
IOTLB flushes of the virtualized s390 IOMMU are used by KVM and z/VM to
synchronize their IOMMU shadow tables thus making them more expensive.
This then applies to all pass-through PCI devices without their drivers
knowing about the IOMMU being virtualized like that. But yes of course
there could be cases where the device driver knows better.

Thanks,
Niklas