RE: [PATCH v10 5/6] iommu: Support mm PASID 1:n with sva domains

[Zhang, Tina]

Hi,

> -----Original Message-----
> From: Zhangfei Gao <zhangfei.gao@xxxxxxxxxx>
> Sent: Wednesday, February 21, 2024 10:45 AM
> To: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Cc: Zhang, Tina <tina.zhang@xxxxxxxxx>; iommu@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; David Woodhouse <dwmw2@xxxxxxxxxxxxx>; Joerg
> Roedel <joro@xxxxxxxxxx>; Will Deacon <will@xxxxxxxxxx>; Robin Murphy
> <robin.murphy@xxxxxxx>; Jason Gunthorpe <jgg@xxxxxxxx>; Tian, Kevin
> <kevin.tian@xxxxxxxxx>; Nicolin Chen <nicolinc@xxxxxxxxxx>; Michael Shavit
> <mshavit@xxxxxxxxxx>; Vasant Hegde <vasant.hegde@xxxxxxx>; Jason
> Gunthorpe <jgg@xxxxxxxxxx>; Jean-Philippe Brucker <jean-
> philippe@xxxxxxxxxx>; Hao Fang <fanghao11@xxxxxxxxxx>
> Subject: Re: [PATCH v10 5/6] iommu: Support mm PASID 1:n with sva
> domains
> 
> On Wed, 21 Feb 2024 at 10:06, Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> >
> > On 2024/2/21 9:28, Zhangfei Gao wrote:
> > > On Wed, 21 Feb 2024 at 07:58, Zhang, Tina<tina.zhang@xxxxxxxxx>
> wrote:
> > >
> > >>>>   struct iommu_sva *iommu_sva_bind_device(struct device *dev,
> > >>>> struct mm_struct *mm)  {
> > >>>> +       struct iommu_mm_data *iommu_mm;
> > >>>>          struct iommu_domain *domain;
> > >>>>          struct iommu_sva *handle;
> > >>>>          int ret;
> > >>>>
> > >>>> +       mutex_lock(&iommu_sva_lock);
> > >>>> +
> > >>>>          /* Allocate mm->pasid if necessary. */
> > >>>> -       ret = iommu_sva_alloc_pasid(mm, dev);
> > >>>> -       if (ret)
> > >>>> -               return ERR_PTR(ret);
> > >>>> +       iommu_mm = iommu_alloc_mm_data(mm, dev);
> > >>>> +       if (IS_ERR(iommu_mm)) {
> > >>>> +               ret = PTR_ERR(iommu_mm);
> > >>>> +               goto out_unlock;
> > >>>> +       }
> > >>>>
> > >>>>          handle = kzalloc(sizeof(*handle), GFP_KERNEL);
> > >>>> -       if (!handle)
> > >>>> -               return ERR_PTR(-ENOMEM);
> > >>>> -
> > >>>> -       mutex_lock(&iommu_sva_lock);
> > >>>> -       /* Search for an existing domain. */
> > >>>> -       domain = iommu_get_domain_for_dev_pasid(dev, mm->pasid,
> > >>>> -                                               IOMMU_DOMAIN_SVA);
> > >>>> -       if (IS_ERR(domain)) {
> > >>>> -               ret = PTR_ERR(domain);
> > >>>> +       if (!handle) {
> > >>>> +               ret = -ENOMEM;
> > >>>>                  goto out_unlock;
> > >>>>          }
> > >>>>
> > >>>> -       if (domain) {
> > >>>> -               domain->users++;
> > >>>> -               goto out;
> > >>> Our multi bind test case broke since 6.8-rc1.
> > >>> The test case can use same domain & pasid, return different
> > >>> handle,
> > >>> 6.7 simply  domain->users ++ and return.
> > >>>
> > >>>> +       /* Search for an existing domain. */
> > >>>> +       list_for_each_entry(domain, &mm->iommu_mm->sva_domains,
> > >>>> + next)
> > >>> {
> > >>>> +               ret = iommu_attach_device_pasid(domain, dev,
> > >>>> + iommu_mm->pasid);
> > >>> Now iommu_attach_device_pasid return BUSY since the same pasid.
> > >>> And then iommu_sva_bind_device attach ret=-16
> > >> Sounds like the test case tries to bind a device to a same mm multiple
> times without unbinding the device and the expectation is that it can always
> return a valid handle to pass the test. Right?
> > > Yes
> > >
> > > The device can bind to the same mm multi-times and return different
> > > handle, Since the refcount, no need to unbind and bind sequently,
> > > The unbind can happen later with the handle.
> >
> > Is there any real use case to bind an mm to the pasid of a device
> > multiple times? If there are cases, is it better to handle this in the
> > uacce driver?
> 
> Yes, it is required for multi-thread, the device can provide multi-queue to
> speed up.
> 
> >
> >  From iommu core's perspective, it doesn't make sense to attach the
> > same domain to the same device (or pasid) multiple times.
> 
> But is it the refcount domain->user++ used for?
> Is there any reason not doing this.
The domain->user is a refcount of the devices (or iommu group) attached to the domain.  IOMMU core needs to keep this refcount to ensure that a sva domain will be released when no device uses it.

Regards,
-Tina

> 
> Thanks