linux-kernel - Re: [PATCH v6 02/10] iommu: Remove sva handle list

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Message-ID: <20240612130554.GR791043@ziepe.ca>
Date: Wed, 12 Jun 2024 10:05:54 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Baolu Lu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Nicolin Chen <nicolinc@...dia.com>,
	"Liu, Yi L" <yi.l.liu@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Joel Granados <j.granados@...sung.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 02/10] iommu: Remove sva handle list

On Fri, Jun 07, 2024 at 09:35:23AM +0000, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@...ux.intel.com>
> > Sent: Thursday, June 6, 2024 2:07 PM
> > 
> > On 6/5/24 4:15 PM, Tian, Kevin wrote:
> > >> From: Lu Baolu <baolu.lu@...ux.intel.com>
> > >> Sent: Monday, May 27, 2024 12:05 PM
> > >>
> > >> -	list_for_each_entry(handle, &mm->iommu_mm->sva_handles,
> > >> handle_item) {
> > >> -		if (handle->dev == dev) {
> > >> -			refcount_inc(&handle->users);
> > >> -			mutex_unlock(&iommu_sva_lock);
> > >> -			return handle;
> > >> -		}
> > >> +	/* A bond already exists, just take a reference`. */
> > >> +	attach_handle = iommu_attach_handle_get(group, iommu_mm-
> > >>> pasid, IOMMU_DOMAIN_SVA);
> > >> +	if (!IS_ERR(attach_handle)) {
> > >> +		handle = container_of(attach_handle, struct iommu_sva,
> > >> handle);
> > >> +		refcount_inc(&handle->users);
> > >> +		mutex_unlock(&iommu_sva_lock);
> > >> +		return handle;
> > >>   	}
> > >
> > > It's counter-intuitive to move forward when an error is returned.
> > >
> > > e.g. if it's -EBUSY indicating the pasid already used for another type then
> > > following attempts shouldn't been tried.

Yes, it looks like iommu_sva_bind_device() should fail with EBUSY if
the PASID is already in use and is not exactly the same SVA as being
asked for here.

It will eventually do this naturally when iommu_attach_device_pasid()
is called with an in-use PASID, but may as well do it here for
clarity.

Also, is there a missing test for the same mm too?

I'd maybe change iommu_attach_handle() to return NULL if there is no
handle and then write it like:

if (IS_ERR(attach_handle) && PTR_ERR(attach_handle) != -ENOENT) {
	ret = PTR_ERR(attach_handle);
	goto out_unlock;
}

if (!IS_ERR(attach_handle) && attach_handle->domain->mm == mm) {
   /* Can re-use the existing SVA attachment */
}

> > > Does it suggest that having the caller to always provide a handle
> > > makes more sense?

I was thinking no just to avoid memory allocation.. But how does the
caller not provide a handle? My original draft of this concept used an
XA_MARK to indicate if the xarray pointed to a handle or a domain

This seems to require the handle:

-	curr = xa_cmpxchg(&group->pasid_array, pasid, NULL, domain, GFP_KERNEL);
-	if (curr) {
-		ret = xa_err(curr) ? : -EBUSY;
+	ret = xa_insert(&group->pasid_array, pasid, handle, GFP_KERNEL);

Confused.

> > I'm neutral on this since only sva bind and iopf path delivery currently
> > require an attach handle.
> 
> let's hear Jason's opinion.

At least iommu_attach_handle_get() should not return NULL if there is
no handle, it should return EBUSY as though it couldn't match the
type.

Jason