linux-kernel - Re: [PATCH v2 02/19] iommufd/viommu: Add IOMMUFD_OBJ_VIOMMU and IOMMU_VIOMMU

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Message-ID: <ZtiY9gE9YDbgJyRN@Asurada-Nvidia>
Date: Wed, 4 Sep 2024 10:29:26 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Baolu Lu <baolu.lu@...ux.intel.com>, <kevin.tian@...el.com>,
	<will@...nel.org>, <joro@...tes.org>, <suravee.suthikulpanit@....com>,
	<robin.murphy@....com>, <dwmw2@...radead.org>, <shuah@...nel.org>,
	<linux-kernel@...r.kernel.org>, <iommu@...ts.linux.dev>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kselftest@...r.kernel.org>,
	<eric.auger@...hat.com>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>, <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 02/19] iommufd/viommu: Add IOMMUFD_OBJ_VIOMMU and
 IOMMU_VIOMMU_ALLOC ioctl

On Wed, Sep 04, 2024 at 01:26:21PM -0300, Jason Gunthorpe wrote:
> On Sun, Sep 01, 2024 at 10:27:09PM -0700, Nicolin Chen wrote:
> > On Sun, Sep 01, 2024 at 10:39:17AM +0800, Baolu Lu wrote:
> > > On 2024/8/28 0:59, Nicolin Chen wrote:
> > > > +int iommufd_viommu_alloc_ioctl(struct iommufd_ucmd *ucmd)
> > > > +{
> > > > +     struct iommu_viommu_alloc *cmd = ucmd->cmd;
> > > > +     struct iommufd_hwpt_paging *hwpt_paging;
> > > > +     struct iommufd_viommu *viommu;
> > > > +     struct iommufd_device *idev;
> > > > +     int rc;
> > > > +
> > > > +     if (cmd->flags)
> > > > +             return -EOPNOTSUPP;
> > > > +
> > > > +     idev = iommufd_get_device(ucmd, cmd->dev_id);
> > > 
> > > Why does a device reference count is needed here? When is this reference
> > > count released after the VIOMMU is allocated?
> > 
> > Hmm, it was used to get dev->iommu->iommu_dev to pin the VIOMMU to
> > a physical IOMMU instance (in v1). Jason suggested to remove that,
> > yet I didn't realize that this idev is now completely useless.
> > 
> > With that being said, a parent HWPT could be shared across VIOMUs
> > allocated for the same VM. So, I think we do need a dev pointer to
> > know which physical instance the VIOMMU allocates for, especially
> > for a driver-managed VIOMMU.
> 
> Eventually you need a way to pin the physical iommu, without pinning
> any idevs. Not sure how best to do that

Just trying to clarify "without pinning any idevs", does it mean
we shouldn't pass in an idev_id to get dev->iommu->iommu_dev?

Otherwise, iommu_probe_device_lock and iommu_device_lock in the
iommu.c are good enough to lock dev->iommu and iommu->list. And
I think we just need an iommu helper refcounting the dev_iommu
(or iommu_device) as we previously discussed.

Thanks
Nicolin