linux-kernel - Re: [PATCH v10 08/12] iommu/sva: Refactoring iommu_sva_bind/unbind

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Date:   Mon, 25 Jul 2022 11:46:05 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        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>,
        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-kernel@...r.kernel.org
Subject: Re: [PATCH v10 08/12] iommu/sva: Refactoring
 iommu_sva_bind/unbind_device()

On Mon, Jul 25, 2022 at 10:52:40AM +0100, Jean-Philippe Brucker wrote:
> > The iommu core provides the interface to retrieve attached domain with a
> > {device, pasid} pair. Therefore in the smmuv3 driver, the set_dev_pasid
> > could do like this:
> 
> Thanks for the example, yes I can do something like this. I maintain that
> attach+detach is clearer, but as long as it can be made to work, fine by
> me

Except it is not clearer, because there isn't actually a detatch in
our model - many things already got messed up in the non-pasid case
because of this confusing assumption.

We have only a "set" operation and set moves between any two domain
configurations.

You don't need to call attach/detach pairs, just repeated attaches,
which is how the normal path works. detach is called in the legacy
flow for the NULL domain

So, creating a pair invites the wrong idea that they actually are a
pair.

> > The check of "(!domain || domain->type == IOMMU_DOMAIN_UNMANAGED)" looks
> > odd, but could get cleaned up after a real blocking domain is added.
> > Then, we can simply check "domain->type == IOMMU_DOMAIN_BLOCKING".

So this is probably a good reason enough not to do it yet, though it
would be nice to get a proper blocking domain concept in the SMMU
driver to support VFIO, it could be done later.

Jason