Re: [PATCH v13 14/20] drm/amdgpu, arm64: untag user pointers in amdgpu_ttm_tt_get_user_pages

[Andrey Konovalov <andreyknvl@google.com>]

On Mon, Mar 25, 2019 at 11:21 PM Kuehling, Felix <Felix.Kuehling@amd.com> wrote:
>
> On 2019-03-20 10:51 a.m., Andrey Konovalov wrote:
> > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > pass tagged user pointers (with the top byte set to something else other
> > than 0x00) as syscall arguments.
> >
> > amdgpu_ttm_tt_get_user_pages() uses provided user pointers for vma
> > lookups, which can only by done with untagged pointers.
> >
> > Untag user pointers in this function.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > index 73e71e61dc99..891b027fa33b 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > @@ -751,10 +751,11 @@ int amdgpu_ttm_tt_get_user_pages(struct ttm_tt *ttm, struct page **pages)
> >                * check that we only use anonymous memory to prevent problems
> >                * with writeback
> >                */
> > -             unsigned long end = gtt->userptr + ttm->num_pages * PAGE_SIZE;
> > +             unsigned long userptr = untagged_addr(gtt->userptr);
> > +             unsigned long end = userptr + ttm->num_pages * PAGE_SIZE;
> >               struct vm_area_struct *vma;
> >
> > -             vma = find_vma(mm, gtt->userptr);
> > +             vma = find_vma(mm, userptr);
> >               if (!vma || vma->vm_file || vma->vm_end < end) {
> >                       up_read(&mm->mmap_sem);
> >                       return -EPERM;
>
> We'll need to be careful that we don't break your change when the
> following commit gets applied through drm-next for Linux 5.2:
>
> https://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-next-5.2-wip&id=915d3eecfa23693bac9e54cdacf84fb4efdcc5c4
>
> Would it make sense to apply the untagging in amdgpu_ttm_tt_set_userptr
> instead? That would avoid this conflict and I think it would clearly put
> the untagging into the user mode code path where the tagged pointer
> originates.
>
> In amdgpu_gem_userptr_ioctl and amdgpu_amdkfd_gpuvm.c (init_user_pages)
> we also set up an MMU notifier with the (tagged) pointer from user mode.
> That should probably also use the untagged address so that MMU notifiers
> for the untagged address get correctly matched up with the right BO. I'd
> move the untagging further up the call stack to cover that. For the GEM
> case I think amdgpu_gem_userptr_ioctl would be the right place. For the
> KFD case, I'd do this in amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu.

Will do in v14, thanks a lot for looking at this!

Is this applicable to the radeon driver (drivers/gpu/drm/radeon) as
well? It seems to be using very similar structure.

>
> Regards,
>    Felix
>