RE: [PATCH V4 1/3] iommu: Add support to change default domain of an iommu group

["Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>]

Hi Joerg,

Replying again because I noticed that I couldn't find this mail in the external iommu mailing list while I was able to find your comments on my patch. Also, could you please answer my two questions below?

> -----Original Message-----
> From: iommu <iommu-bounces@lists.linux-foundation.org> On Behalf Of
> Prakhya, Sai Praneeth
> Sent: Tuesday, June 30, 2020 8:04 PM
> To: Joerg Roedel <joro@8bytes.org>
> Cc: Raj, Ashok <ashok.raj@intel.com>; Will Deacon <will.deacon@arm.com>;
> iommu@lists.linux-foundation.org; Robin Murphy <robin.murphy@arm.com>;
> Christoph Hellwig <hch@lst.de>
> Subject: RE: [PATCH V4 1/3] iommu: Add support to change default domain of
> an iommu group
> 
> Hi Joerg,
> 
> > -----Original Message-----
> > From: Joerg Roedel <joro@8bytes.org>
> > Sent: Tuesday, June 30, 2020 2:16 AM
> > To: Prakhya, Sai Praneeth <sai.praneeth.prakhya@intel.com>
> > Cc: iommu@lists.linux-foundation.org; Christoph Hellwig <hch@lst.de>;
> > Raj, Ashok <ashok.raj@intel.com>; Will Deacon <will.deacon@arm.com>;
> > Lu Baolu <baolu.lu@linux.intel.com>; Mehta, Sohil
> > <sohil.mehta@intel.com>; Robin Murphy <robin.murphy@arm.com>; Jacob
> > Pan <jacob.jun.pan@linux.intel.com>
> > Subject: Re: [PATCH V4 1/3] iommu: Add support to change default
> > domain of an iommu group
> >
> > On Thu, Jun 04, 2020 at 06:32:06PM -0700, Sai Praneeth Prakhya wrote:
> > > +static int iommu_change_dev_def_domain(struct iommu_group *group,
> > > +int
> > > +type) {
> > > +	struct iommu_domain *prev_dom;
> > > +	struct group_device *grp_dev;
> > > +	const struct iommu_ops *ops;
> > > +	int ret, dev_def_dom;
> > > +	struct device *dev;
> > > +
> > > +	if (!group)
> > > +		return -EINVAL;
> > > +
> > > +	mutex_lock(&group->mutex);
> > > +
> > > +	if (group->default_domain != group->domain) {
> > > +		pr_err_ratelimited("Group assigned to user level for direct
> > > +access\n");
> >
> > Make this message: "Group not assigned to default domain\n".
> 
> Sure! I will change it
> 
> > > +		ret = -EBUSY;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/*
> > > +	 * iommu group wasn't locked while acquiring device lock in
> > > +	 * iommu_group_store_type(). So, make sure that the device count
> > hasn't
> > > +	 * changed while acquiring device lock.
> > > +	 *
> > > +	 * Changing default domain of an iommu group with two or more
> > devices
> > > +	 * isn't supported because there could be a potential deadlock. Consider
> > > +	 * the following scenario. T1 is trying to acquire device locks of all
> > > +	 * the devices in the group and before it could acquire all of them,
> > > +	 * there could be another thread T2 (from different sub-system and use
> > > +	 * case) that has already acquired some of the device locks and might be
> > > +	 * waiting for T1 to release other device locks.
> > > +	 */
> > > +	if (iommu_group_device_count(group) != 1) {
> > > +		pr_err_ratelimited("Cannot change default domain of a group
> > with
> > > +two or more devices\n");
> >
> > "Can not change default domain: Group has more than one device\n"
> 
> Ok.. make sense. I will change this.
> 
> > > +		ret = -EINVAL;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/* Since group has only one device */
> > > +	list_for_each_entry(grp_dev, &group->devices, list)
> > > +		dev = grp_dev->dev;
> > > +
> > > +	prev_dom = group->default_domain;
> > > +	if (!prev_dom || !prev_dom->ops || !prev_dom->ops-
> > >def_domain_type) {
> > > +		pr_err_ratelimited("'def_domain_type' call back isn't
> > > +registered\n");
> >
> > This message isn't needed.
> 
> Ok. I will remove it.
> 
> > > +	ret = __iommu_attach_device(group->default_domain, dev);
> > > +	if (ret)
> > > +		goto free_new_domain;
> > > +
> > > +	group->domain = group->default_domain;
> > > +
> > > +	ret = iommu_create_device_direct_mappings(group, dev);
> > > +	if (ret)
> > > +		goto free_new_domain;
> >
> > You need to create the direct mappings before you attach the device to
> > the new domain. Otherwise there might be a short time-window where
> > RMRR regions are not mapped.
> 
> Ok.. makes sense. I will change this accordingly.
> 
> > > +static ssize_t iommu_group_store_type(struct iommu_group *group,
> > > +				      const char *buf, size_t count) {
> > > +	struct group_device *grp_dev;
> > > +	struct device *dev;
> > > +	int ret, req_type;
> > > +
> > > +	if (!capable(CAP_SYS_ADMIN) || !capable(CAP_SYS_RAWIO))
> > > +		return -EACCES;
> > > +
> > > +	if (WARN_ON(!group))
> > > +		return -EINVAL;
> > > +
> > > +	if (sysfs_streq(buf, "identity"))
> > > +		req_type = IOMMU_DOMAIN_IDENTITY;
> > > +	else if (sysfs_streq(buf, "DMA"))
> > > +		req_type = IOMMU_DOMAIN_DMA;
> > > +	else if (sysfs_streq(buf, "auto"))
> > > +		req_type = 0;
> > > +	else
> > > +		return -EINVAL;
> > > +
> > > +	/*
> > > +	 * Lock/Unlock the group mutex here before device lock to
> > > +	 * 1. Make sure that the iommu group has only one device (this is a
> > > +	 *    prerequisite for step 2)
> > > +	 * 2. Get struct *dev which is needed to lock device
> > > +	 */
> > > +	mutex_lock(&group->mutex);
> > > +	if (iommu_group_device_count(group) != 1) {
> > > +		mutex_unlock(&group->mutex);
> > > +		pr_err_ratelimited("Cannot change default domain of a group
> > with two or more devices\n");
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	/* Since group has only one device */
> > > +	list_for_each_entry(grp_dev, &group->devices, list)
> > > +		dev = grp_dev->dev;
> >
> > Please use list_first_entry().
> 
> Ok.
> 
> > You also need to take a reference with get_device() and then drop the
> > group->mutex.
> 
> Sure! I will change it.
> 
> > After device_lock() you need to verify that the device is still in the
> > same group and that the group has still only one device in it.

Q1:
> Presently, iommu_change_dev_def_domain() checks if the iommu group still has
> only one device or not. Hence, checking if iommu group has one device or not is
> done twice, once before taking device_lock() and the other, after taking
> device_lock().
> 
> I agree that the code isn't checking if the iommu group still has the _same_
> device or not.
> One way, I could think of doing it is by storing "dev" temporarily and checking
> for it.
> Do you think that's ok? Or would you rather suggest something else?
> 
> > Then you can call down to
> > iommu_change_dev_def_domain() which does not need to take the group-
> > mutex by itself.

Q2:
> The reason for taking iommu_group->mutex in the beginning of
> iommu_change_dev_def_domain() is that the function
> 
> 1. Checks if the group is being directly used by user level drivers (i.e. if (group-
> >default_domain != group->domain))
> 
> 2. Uses iommu_ops
> (prev_dom = group->default_domain;
> if (!prev_dom || !prev_dom->ops || !prev_dom->ops->def_domain_type))
> 
> 3. Sets iomu_group->domain to iommu_group->default_domain
> 
> I wanted to make sure that iommu_group->domain and iommu_group-
> >default_domain aren't changed by some other thread while this thread is
> working on it. So, please let me know if I misunderstood something.
> 
> Regards,
> Sai
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