linux-kernel - Re: [RFC PATCH v2 08/10] vfio/type1: Add domain at(de)taching group helpers

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Date:   Wed, 12 Sep 2018 18:54:26 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
Cc:     "kevin.tian@...el.com" <kevin.tian@...el.com>,
        "ashok.raj@...el.com" <ashok.raj@...el.com>,
        "tiwei.bie@...el.com" <tiwei.bie@...el.com>,
        "sanjay.k.kumar@...el.com" <sanjay.k.kumar@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yi.y.sun@...el.com" <yi.y.sun@...el.com>,
        "jacob.jun.pan@...el.com" <jacob.jun.pan@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH v2 08/10] vfio/type1: Add domain at(de)taching group
 helpers

On 12/09/2018 06:02, Lu Baolu wrote:
> Hi,
> 
> On 09/11/2018 12:23 AM, Jean-Philippe Brucker wrote:
>> On 30/08/2018 05:09, Lu Baolu wrote:
>>> +static int vfio_detach_aux_domain(struct device *dev, void *data)
>>> +{
>>> +	struct iommu_domain *domain = data;
>>> +
>>> +	vfio_mdev_set_aux_domain(dev, NULL);
>>> +	iommu_detach_device(domain, dev->parent);
>>
>> I think that's only going to work for vt-d, which doesn't use a
>> default_domain. For other drivers, iommu.c ends up calling
>> domain->ops->attach_dev(default_domain, dev) here, which isn't what we want.
> 
> This doesn't break any functionality. It takes effect only if iommu
> hardware supports finer granular translation and advertises it through
> the API.>
>> That was my concern with reusing attach/detach_dev callbacks for PASID
>> management. The attach_dev code of IOMMU drivers already has to deal
>> with toggling between default and unmanaged domain. Dealing with more
>> state transitions in the same path is going to be difficult.
> 
> Let's consider it from the API point of view.
> 
> We have iommu_a(de)ttach_device() APIs to attach or detach a domain
> to/from a device. We should avoid applying a limitation of "these are
> only for single domain case, for multiple domains, use another API".

That's an idealized view of the API, the actual semantics aren't as
simple. For IOMMU drivers that implement IOMMU_DOMAIN_DMA in their
domain_alloc operation (Arm SMMU, AMD IOMMU, ...), attach_dev attaches
an unmanaged domain, detach_dev reattaches the default DMA domain, and
the detach_dev IOMMU op is not called. We can't change that now, so it's
difficult to add more functionality to attach_dev and detach_dev.

Thanks,
Jean