Message-ID: <d8fdf1a5-3405-d449-bf94-7e2c19da8d15@linux.ibm.com>
Date:   Fri, 17 Feb 2023 09:40:01 -0500
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Subject: Re: [PATCH v6 5/6] iommu/dma: Allow a single FQ in addition to
 per-CPU FQs
Content-Language: en-US
To:     Niklas Schnelle <schnelle@linux.ibm.com>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Wenjia Zhang <wenjia@linux.ibm.com>
Cc:     Gerd Bayer <gbayer@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>, iommu@lists.linux.dev,
        linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        svens@linux.ibm.com, linux-kernel@vger.kernel.org,
        Julian Ruess <julianr@linux.ibm.com>
References: <20230215120327.947336-1-schnelle@linux.ibm.com>
 <20230215120327.947336-6-schnelle@linux.ibm.com>
From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <20230215120327.947336-6-schnelle@linux.ibm.com>
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On 2/15/23 7:03 AM, Niklas Schnelle wrote:
> In some virtualized environments, including s390 paged memory guests,
> IOTLB flushes are used to update IOMMU shadow tables. Due to this, they
> are much more expensive than in typical bare metal environments or
> non-paged s390 guests. In addition they may parallelize more poorly in
> virtualized environments. This changes the trade off for flushing IOVAs
> such that minimizing the number of IOTLB flushes trumps any benefit of
> cheaper queuing operations or increased paralellism.
> 
> In this scenario per-CPU flush queues pose several problems. Firstly
> per-CPU memory is often quite limited prohibiting larger queues.
> Secondly collecting IOVAs per-CPU but flushing via a global timeout
> reduces the number of IOVAs flushed for each timeout especially on s390
> where PCI interrupts may not be bound to a specific CPU.
> 
> Let's introduce a single flush queue mode that reuses the same queue
> logic but only allocates a single global queue. This mode can be
> selected as a flag bit in a new dma_iommu_options struct which can be
> modified from its defaults by IOMMU drivers implementing a new
> ops.tune_dma_iommu() callback. As a first user the s390 IOMMU driver
> selects the single queue mode if IOTLB flushes are needed on map which
> indicates shadow table use. With the unchanged small FQ size and
> timeouts this setting is worse than per-CPU queues but a follow up patch
> will make the FQ size and timeout variable. Together this allows the
> common IOVA flushing code to more closely resemble the global flush
> behavior used on s390's previous internal DMA API implementation.
> 
> Link: https://lore.kernel.org/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@arm.com/
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390