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

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Message-ID: <ZtVMrXXESy/RfWVi@Asurada-Nvidia>
Date: Sun, 1 Sep 2024 22:27:09 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
CC: <jgg@...dia.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 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.

Perhaps we should add back the iommu_dev and properly refcount it.

Thanks
Nicolin