netdev - Re: [PATCH v12 5/6] iommu/dma: Allow a single FQ in addition to per-CPU FQs

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Message-ID: <c5ac1459d2967c17e472d1c51d5017beb39e4714.camel@linux.ibm.com>
Date: Mon, 11 Sep 2023 14:06:29 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Joerg Roedel <joro@...tes.org>, Matthew Rosato <mjrosato@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, Wenjia Zhang <wenjia@...ux.ibm.com>,
        Robin
 Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Gerd Bayer <gbayer@...ux.ibm.com>, Julian Ruess <julianr@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Alexandra Winter
 <wintera@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik
 <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian
 Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle
 <svens@...ux.ibm.com>,
        Suravee Suthikulpanit
 <suravee.suthikulpanit@....com>,
        Hector Martin <marcan@...can.st>, Sven
 Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
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 <angelogioacchino.delregno@...labora.com>,
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        Thierry Reding
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        Krishna Reddy <vdumpa@...dia.com>,
        Jonathan
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        Jonathan Corbet <corbet@....net>, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-tegra@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v12 5/6] iommu/dma: Allow a single FQ in addition to
 per-CPU FQs

On Fri, 2023-08-25 at 12:11 +0200, 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 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 is selected by
> dma-iommu if a newly introduced .shadow_on_flush flag is set in struct
> dev_iommu. As a first user the s390 IOMMU driver sets this flag during
> probe_device. 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/all/9a466109-01c5-96b0-bf03-304123f435ee@arm.com/
> Acked-by: Robin Murphy <robin.murphy@....com>
> Reviewed-by: Matthew Rosato <mjrosato@...ux.ibm.com> #s390
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>  drivers/iommu/dma-iommu.c  | 168 ++++++++++++++++++++++++++++++++++-----------
>  drivers/iommu/s390-iommu.c |   3 +
>  include/linux/iommu.h      |   2 +
>  3 files changed, 134 insertions(+), 39 deletions(-)
> 
> 
---8<---
>  
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 182cc4c71e62..c3687e066ed7 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -409,6 +409,7 @@ struct iommu_fault_param {
>   * @priv:	 IOMMU Driver private data
>   * @max_pasids:  number of PASIDs this device can consume
>   * @attach_deferred: the dma domain attachment is deferred
> + * @shadow_on_flush: IOTLB flushes are used to sync shadow tables
>   *
>   * TODO: migrate other per device data pointers under iommu_dev_data, e.g.
>   *	struct iommu_group	*iommu_group;
> @@ -422,6 +423,7 @@ struct dev_iommu {
>  	void				*priv;
>  	u32				max_pasids;
>  	u32				attach_deferred:1;
> +	u32				shadow_on_flush:1;

This causes a merge conflict with a48ce36e2786f ("iommu: Prevent
RESV_DIRECT devices from blocking domains"), The resolution is trivial
though in that shadow_on_flush:1 can just be added after (or before)
require_direct:1. @Joro do you want me to sent a version with this
resolution regardless or will you resolve this when applying?

>  };
>  
>  int iommu_device_register(struct iommu_device *iommu,
>