linux-kernel - Re: [PATCH v12 12/17] arm-smmu-v3/sva: Add SVA domain support

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Message-ID: <4911af2a-134c-bb3b-dfa4-3dc7b69e11a3@linux.intel.com>
Date:   Tue, 30 Aug 2022 10:04:52 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v12 12/17] arm-smmu-v3/sva: Add SVA domain support

On 2022/8/30 01:29, Jason Gunthorpe wrote:
> On Sun, Aug 28, 2022 at 09:57:21PM +0800, Baolu Lu wrote:
>> On 2022/8/26 22:56, Jason Gunthorpe wrote:
>>> On Fri, Aug 26, 2022 at 08:11:36PM +0800, Lu Baolu wrote:
>>>
>>>> +static const struct iommu_domain_ops arm_smmu_sva_domain_ops = {
>>>> +	.set_dev_pasid		= arm_smmu_sva_set_dev_pasid,
>>> Do we want to permit drivers to not allow a SVA domain to be set on a
>>> RID?
>>>
>>> It seems like a weird restriction to me
>> Conceptually as long as the page table is compatible and user pages are
>> pinned (or I/O page fault is supported), the device drivers are valid to
>> set SVA domain to a RID. But I don't see a real use case as far as I can
>> see.
> It may be interesting for something like DPDK type applications where
> having the entire process address space mapped SVA to the device could
> be quite nice.
> 
> You, currently, give up interrupts, but perhaps that is solvable in some
> way.
> 
> So, IDK.. I wouldn't dismiss it entirely but I wouldn't do a bunch of
> work to support it either.

Then we can do this through the set_dev callback, as it's the right
callback to set a domain to the RID, right? Not sure whether it worth a
new type of domain. The current implementation doesn't prevent us from
achieving this in the future anyway.

> 
>> A reasonable use case is sharing EPT between KVM and IOMMU. That demands
>> a new type of domain and implements its own .set_dev for page table
>> attachment.
> Not everything is virtualization:)

Yes. Fair enough. :-)

Best regards,
baolu