linux-kernel - Re: [PATCH v2 08/16] iommu: define and export iommu_access_remote

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Message-ID: <ZBhhEK1V+u2oV2Ll@infradead.org>
Date:   Mon, 20 Mar 2023 06:35:12 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Baolu Lu <baolu.lu@...ux.intel.com>, Vinod Koul <vkoul@...nel.org>,
        Dave Jiang <dave.jiang@...el.com>, dmaengine@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alistair Popple <apopple@...dia.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>, iommu@...ts.linux.dev
Subject: Re: [PATCH v2 08/16] iommu: define and export
 iommu_access_remote_vm()

On Tue, Mar 07, 2023 at 09:55:28AM -0800, Fenghua Yu wrote:
> > 
> > I don't quite follow here. Isn't I/O page fault already supported?
> 
> The following patch 9 in this series explains in details why IDXD device
> cannot use page fault to write to user memory: https://lore.kernel.org/dmaengine/20230306163138.587484-10-fenghua.yu@intel.com/
> 
> "DSA supports page fault handling through PRS. However, the DMA engine
> that's processing the descriptor is blocked until the PRS response is
> received. Other workqueues sharing the engine are also blocked.
> Page fault handing by the driver with PRS disabled can be used to
> mitigate the stalling.
> 
> With PRS disabled while ATS remain enabled, DSA handles page faults on
> a completion record by reporting an event in the event log. In this
> instance, the descriptor is completed and the event log contains the
> completion record address and the contents of the completion record."

This seems like a completely broken I/O model, and I don't think Linux
should support this model when it requires operations like
access_remote_vm.