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

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Message-ID: <20240904162621.GN3915968@nvidia.com>
Date: Wed, 4 Sep 2024 13:26:21 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...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 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

Jason