linux-kernel - Re: [PATCH v6 08/12] iommu/sva: Use attach/detach_pasid

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Message-ID: <20220511145319.GZ49344@nvidia.com>
Date:   Wed, 11 May 2022 11:53:19 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     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>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 08/12] iommu/sva: Use attach/detach_pasid_dev in SVA
 interfaces

On Wed, May 11, 2022 at 03:21:31PM +0800, Baolu Lu wrote:
> On 2022/5/10 23:23, Jason Gunthorpe wrote:
> > On Tue, May 10, 2022 at 02:17:34PM +0800, Lu Baolu wrote:
> > 
> > > +/**
> > > + * iommu_sva_bind_device() - Bind a process address space to a device
> > > + * @dev: the device
> > > + * @mm: the mm to bind, caller must hold a reference to mm_users
> > > + * @drvdata: opaque data pointer to pass to bind callback
> > > + *
> > > + * Create a bond between device and address space, allowing the device to access
> > > + * the mm using the returned PASID. If a bond already exists between @device and
> > > + * @mm, it is returned and an additional reference is taken. Caller must call
> > > + * iommu_sva_unbind_device() to release each reference.
> > > + *
> > > + * iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) must be called first, to
> > > + * initialize the required SVA features.
> > > + *
> > > + * On error, returns an ERR_PTR value.
> > > + */
> > > +struct iommu_sva *
> > > +iommu_sva_bind_device(struct device *dev, struct mm_struct *mm, void *drvdata)
> > > +{
> > > +	int ret = -EINVAL;
> > > +	struct iommu_sva *handle;
> > > +	struct iommu_domain *domain;
> > > +
> > > +	/*
> > > +	 * TODO: Remove the drvdata parameter after kernel PASID support is
> > > +	 * enabled for the idxd driver.
> > > +	 */
> > > +	if (drvdata)
> > > +		return ERR_PTR(-EOPNOTSUPP);
> > 
> > Why is this being left behind? Clean up the callers too please.
> 
> Okay, let me try to.
> 
> > 
> > > +	/* Allocate mm->pasid if necessary. */
> > > +	ret = iommu_sva_alloc_pasid(mm, 1, (1U << dev->iommu->pasid_bits) - 1);
> > > +	if (ret)
> > > +		return ERR_PTR(ret);
> > > +
> > > +	mutex_lock(&iommu_sva_lock);
> > > +	/* Search for an existing bond. */
> > > +	handle = xa_load(&dev->iommu->sva_bonds, mm->pasid);
> > > +	if (handle) {
> > > +		refcount_inc(&handle->users);
> > > +		goto out_success;
> > > +	}
> > 
> > How can there be an existing bond?
> > 
> > dev->iommu is per-device
> > 
> > The device_group_immutable_singleton() insists on a single device
> > group
> > 
> > Basically 'sva_bonds' is the same thing as the group->pasid_array.
> 
> Yes, really.
> 
> > 
> > Assuming we leave room for multi-device groups this logic should just
> > be
> > 
> > 	group = iommu_group_get(dev);
> > 	if (!group)
> > 		return -ENODEV;
> > 
> > 	mutex_lock(&group->mutex);
> > 	domain = xa_load(&group->pasid_array, mm->pasid);
> > 	if (!domain || domain->type != IOMMU_DOMAIN_SVA || domain->mm != mm)
> > 		domain = iommu_sva_alloc_domain(dev, mm);
> > 
> > ?
> 
> Agreed. As a helper in iommu core, how about making it more generic like
> below?

IDK, is there more users of this? AFAIK SVA is the only place that
will be auto-sharing?

> +       mutex_lock(&group->mutex);
> +       domain = xa_load(&group->pasid_array, pasid);
> +       if (domain && domain->type != type)
> +               domain = NULL;
> +       mutex_unlock(&group->mutex);
> +       iommu_group_put(group);
> +
> +       return domain;

This is bad locking, group->pasid_array values cannot be taken outside
the lock.

> > And stick the refcount in the sva_domain
> > 
> > Also, given the current arrangement it might make sense to have a
> > struct iommu_domain_sva given that no driver is wrappering this in
> > something else.
> 
> Fair enough. How about below wrapper?
> 
> +struct iommu_sva_domain {
> +       /*
> +        * Common iommu domain header, *must* be put at the top
> +        * of the structure.
> +        */
> +       struct iommu_domain domain;
> +       struct mm_struct *mm;
> +       struct iommu_sva bond;
> +}
>
> The refcount is wrapped in bond.

I'm still not sure that bond is necessary

But yes, something like that

Jason