linux-kernel - RE: [PATCH v2 07/10] iommufd/device: Make hwpt_list list

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Date:   Fri, 10 Feb 2023 01:46:18 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
Subject: RE: [PATCH v2 07/10] iommufd/device: Make hwpt_list list_add/del
 symmetric

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, February 9, 2023 9:24 PM
> 
> On Thu, Feb 09, 2023 at 03:23:47AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Wednesday, February 8, 2023 5:18 AM
> > >
> > > Because list_del() is together with iopt_table_remove_domain(), it makes
> > > sense to have list_add_tail() together with iopt_table_add_domain().
> > >
> > > Also place the mutex outside the iommufd_device_do_attach() call,
> similar
> > > to what's in the iommufd_device_auto_get_domain() function.
> > >
> > > Co-developed-by: Yi Liu <yi.l.liu@...el.com>
> > > Signed-off-by: Yi Liu <yi.l.liu@...el.com>
> > > Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> > > Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> >
> > shouldn't this be a separate bug fix and backported? double adding a
> > list item would certainly clobber the list...
> 
> AFAIK there is no bug, this is just reorganizing things
> 

there is semantics change.

here is the current code:

	case IOMMUFD_OBJ_HW_PAGETABLE: {
		struct iommufd_hw_pagetable *hwpt =
			container_of(pt_obj, struct iommufd_hw_pagetable, obj);

		rc = iommufd_device_do_attach(idev, hwpt);
		if (rc)
			goto out_put_pt_obj;

		mutex_lock(&hwpt->ioas->mutex);
		list_add_tail(&hwpt->hwpt_item, &hwpt->ioas->hwpt_list);
		mutex_unlock(&hwpt->ioas->mutex);
		break;
	}

Above means every attach to hwpt will try to add the hwpt to the
list tail. Isn't it a bug?

with this patch the hwpt is added to the list only when it's attached
by the first device, i.e. when iopt_table_add_domain() is called.

	if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
		rc = iommu_attach_group(hwpt->domain, idev->group);
		if (rc)
			goto out_iova;

		if (list_empty(&hwpt->devices)) {
			rc = iopt_table_add_domain(&hwpt->ioas->iopt,
						hwpt->domain);
			if (rc)
				goto out_detach;
			list_add_tail(&hwpt->hwpt_item, &hwpt->ioas->hwpt_list);
		}
	}

so it's actually a bug fix.