Messages in this thread | | | Date | Mon, 25 Apr 2022 17:13:02 +0100 | From | Jean-Philippe Brucker <> | Subject | Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit |
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Hi Jacob,
On Mon, Apr 25, 2022 at 08:34:44AM -0700, Jacob Pan wrote: > Hi Jean-Philippe, > > On Mon, 25 Apr 2022 15:26:40 +0100, Jean-Philippe Brucker > <jean-philippe@linaro.org> wrote: > > > On Mon, Apr 25, 2022 at 07:18:36AM -0700, Dave Hansen wrote: > > > On 4/25/22 06:53, Jean-Philippe Brucker wrote: > > > > On Sat, Apr 23, 2022 at 07:13:39PM +0800, zhangfei.gao@foxmail.com > > > > wrote: > > > >>>> On 5.17 > > > >>>> fops_release is called automatically, as well as > > > >>>> iommu_sva_unbind_device. On 5.18-rc1. > > > >>>> fops_release is not called, have to manually call close(fd) > > > >>> Right that's weird > > > >> Looks it is caused by the fix patch, via mmget, which may add > > > >> refcount of fd. > > > > Yes indirectly I think: when the process mmaps the queue, > > > > mmap_region() takes a reference to the uacce fd. That reference is > > > > released either by explicit close() or munmap(), or by exit_mmap() > > > > (which is triggered by mmput()). Since there is an mm->fd dependency, > > > > we cannot add a fd->mm dependency, so no mmget()/mmput() in > > > > bind()/unbind(). > > > > > > > > I guess we should go back to refcounted PASIDs instead, to avoid > > > > freeing them until unbind(). > > > > > > Yeah, this is a bit gnarly for -rc4. Let's just make sure there's > > > nothing else simple we can do. > > > > > > How does the IOMMU hardware know that all activity to a given PASID is > > > finished? That activity should, today, be independent of an mm or a > > > fd's lifetime. > > > > In the case of uacce, it's tied to the fd lifetime: opening an accelerator > > queue calls iommu_sva_bind_device(), which sets up the PASID context in > > the IOMMU. Closing the queue calls iommu_sva_unbind_device() which > > destroys the PASID context (after the device driver stopped all DMA for > > this PASID). > > > For VT-d, it is essentially the same flow except managed by the individual > drivers such as DSA. > If free() happens before unbind(), we deactivate the PASIDs and suppress > faults from the device. When the unbind finally comes, we finalize the > PASID teardown. It seems we have a need for an intermediate state where > PASID is "pending free"?
Yes we do have that state, though I'm not sure we need to make it explicit in the ioasid allocator.
Could we move mm_pasid_drop() to __mmdrop() instead of __mmput()? For Arm we do need to hold the mm_count until unbind(), and mmgrab()/mmdrop() is also part of Lu's rework [1].
Thanks, Jean
[1] https://lore.kernel.org/linux-iommu/20220421052121.3464100-9-baolu.lu@linux.intel.com/
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