lkml.org 
[lkml]   [2022]   [Apr]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit
    Hi Jean-Philippe,

    On Mon, 25 Apr 2022 17:13:02 +0100, Jean-Philippe Brucker
    <jean-philippe@linaro.org> wrote:

    > 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.
    >
    IMHO, making it explicit would fail ioasid_get() on a "pending free" PASID.
    Making free a one-way trip and prevent further complications.

    > 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].
    >
    Yes, I would agree. IIRC, Fenghua's early patch was doing pasid drop
    in mmdrop. Maybe I missed something.

    > Thanks,
    > Jean
    >
    > [1]
    > https://lore.kernel.org/linux-iommu/20220421052121.3464100-9-baolu.lu@linux.intel.com/


    Thanks,

    Jacob

    \
     
     \ /
      Last update: 2022-04-26 00:30    [W:4.291 / U:0.016 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site