[PATCH v6 2/8] drm/ttm: Add ttm_bo_access
Matthew Brost
matthew.brost at intel.com
Wed Nov 6 15:25:00 UTC 2024
On Wed, Nov 06, 2024 at 10:48:45AM +0100, Christian König wrote:
> Am 05.11.24 um 19:35 schrieb Matthew Brost:
> > [SNIP]
> > > Well we spend quite some time removing single page mappings from device
> > > drivers.
> > >
> > > The only remaining use case of ttm_bo_kmap() with just one page is the
> > > ttm_bo_vm_access_kmap() function and I was really hoping to make that one
> > > TTM internal at some point.
> > >
> > This is still static, right? I suppose this exposes this to the outside
> > world though in another place. I asume there is a reason we can't use
> > vmap in ttm_bo_vm_access?
>
> Well no, the point is we don't want to.
>
> There is a huge push from upstream to avoid using kmap/vmap if possible.
>
> > > > > You need a really good justification to bring that back.
> > > > >
> > > > The use case is EuDebugger requires essentially the same functionality
> > > > as ptrace -> vm_access.
> > > Then why don't you use ptrace in the first place?
> > >
> > I think the debugger speaks in GPU address space thus needs to access
> > via the GPU VM -> BO, userptrs.
>
> Exactly that is strictly forbidden. You can't access userptrs through this.
>
My mistake for mentioning userptr—I clearly caused confusion. This patch
itself has nothing to do with userptr; it is accessing a BO. In Xe, a
userptr doesn't have a BO, unlike in AMDGPU, where you have BOs for
userptrs.
The above use case was an example of modifying a GPU program with
breakpoints, speaking in GPU address space rather than CPU address
space. Hence, we cannot use ptrace. Userptr is a possible example, but
that access path in the code is different and, again, has nothing to do
with BO access in this patch.
> That's one of the major reasons why upstream has pushed back on using kmap
> so massively.
Userptr access is not part of this patch—it will be a separate code
path, so this seemingly does not apply.
>
> Can you fully describe your use case? In other words what exactly is your
> debugger trying to do?
See above; I hope I've made this clearer.
Also, I'm not really an expert on Eudebug, as I haven't been involved in
the development aside from reviewing its interaction with the core of
Xe. Any further explanation would likely require me to loop in a
colleague.
>
> > > > TTM mapping non-contiguous VRAM doesn't work unless I'm blind. User BOs
> > > > which the EuDebugger accesses can be non-contiguous, hence the new
> > > > helper.
> > > Then why don't you handle that inside the driver in the first place instead
> > > of going through a TTM midlayer?
> > >
> > Well common code always seems like a good idea to me. Can do this if you
> > insist though.
> >
> > What if I change my new helper ttm_bo_access to be based on vmap for
> > SYSTEM / TT but honestly that seems wasteful too for a temporary
> > access mapping.
>
> Well, I think we need to take a step back. The major question is what is
> your use case and is that use case valid or causes security concerns.
>
> For example userptrs are imported anonymous pages the GPU has a DMA mapping
> for. Re-mapping them into an user address space for debugging or even
> accessing them through the ptrace interface is strictly forbidden.
>
> We already had people trying to do exactly that and it ended not well at
> all.
>
Again, if we can focus on what this patch is doing—accessing a BO, not a
userptr—I think that will help progress here.
To bring things together: "There is a huge push from upstream to avoid
using kmap/vmap if possible." How would you suggest accessing a BO then?
kmap/vmap are used everywhere in the DRM subsystem to access BOs, so I’m
failing to see the problem with adding a simple helper based on existing
code.
Matt
> Regards,
> Christian.
>
> >
> > With this, I strongly prefer the code as is.
> >
> > Matt
> >
> > > Regards,
> > > Christian.
> > >
> > > > Matt
> > > >
> > > > > Regards,
> > > > > Christian.
> > > > >
> > > > > > > Matt
> > > > > > >
> > > > > > > > Reported-by: Christoph Manszewski<christoph.manszewski at intel.com>
> > > > > > > > Suggested-by: Thomas Hellström<thomas.hellstrom at linux.intel.com>
> > > > > > > > Signed-off-by: Matthew Brost<matthew.brost at intel.com>
> > > > > > > > Tested-by: Mika Kuoppala<mika.kuoppala at linux.intel.com>
> > > > > > > > Reviewed-by: Matthew Auld<matthew.auld at intel.com>
> > > > > > > > ---
> > > > > > > > drivers/gpu/drm/ttm/ttm_bo_util.c | 86 +++++++++++++++++++++++++++++++
> > > > > > > > drivers/gpu/drm/ttm/ttm_bo_vm.c | 65 +----------------------
> > > > > > > > include/drm/ttm/ttm_bo.h | 2 +
> > > > > > > > 3 files changed, 89 insertions(+), 64 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > > > > > > index d939925efa81..77e760ea7193 100644
> > > > > > > > --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > > > > > > +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > > > > > > @@ -919,3 +919,89 @@ s64 ttm_lru_walk_for_evict(struct ttm_lru_walk *walk, struct ttm_device *bdev,
> > > > > > > > return progress;
> > > > > > > > }
> > > > > > > > +
> > > > > > > > +static int ttm_bo_access_kmap(struct ttm_buffer_object *bo,
> > > > > > > > + unsigned long offset,
> > > > > > > > + void *buf, int len, int write)
> > > > > > > > +{
> > > > > > > > + unsigned long page = offset >> PAGE_SHIFT;
> > > > > > > > + unsigned long bytes_left = len;
> > > > > > > > + int ret;
> > > > > > > > +
> > > > > > > > + /* Copy a page at a time, that way no extra virtual address
> > > > > > > > + * mapping is needed
> > > > > > > > + */
> > > > > > > > + offset -= page << PAGE_SHIFT;
> > > > > > > > + do {
> > > > > > > > + unsigned long bytes = min(bytes_left, PAGE_SIZE - offset);
> > > > > > > > + struct ttm_bo_kmap_obj map;
> > > > > > > > + void *ptr;
> > > > > > > > + bool is_iomem;
> > > > > > > > +
> > > > > > > > + ret = ttm_bo_kmap(bo, page, 1, &map);
> > > > > > > > + if (ret)
> > > > > > > > + return ret;
> > > > > > > > +
> > > > > > > > + ptr = (void *)ttm_kmap_obj_virtual(&map, &is_iomem) + offset;
> > > > > > > > + WARN_ON_ONCE(is_iomem);
> > > > > > > > + if (write)
> > > > > > > > + memcpy(ptr, buf, bytes);
> > > > > > > > + else
> > > > > > > > + memcpy(buf, ptr, bytes);
> > > > > > > > + ttm_bo_kunmap(&map);
> > > > > > > > +
> > > > > > > > + page++;
> > > > > > > > + buf += bytes;
> > > > > > > > + bytes_left -= bytes;
> > > > > > > > + offset = 0;
> > > > > > > > + } while (bytes_left);
> > > > > > > > +
> > > > > > > > + return len;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +/**
> > > > > > > > + * ttm_bo_access - Helper to access a buffer object
> > > > > > > > + *
> > > > > > > > + * @bo: ttm buffer object
> > > > > > > > + * @offset: access offset into buffer object
> > > > > > > > + * @buf: pointer to caller memory to read into or write from
> > > > > > > > + * @len: length of access
> > > > > > > > + * @write: write access
> > > > > > > > + *
> > > > > > > > + * Utility function to access a buffer object. Useful when buffer object cannot
> > > > > > > > + * be easily mapped (non-contiguous, non-visible, etc...).
> > > > > > > > + *
> > > > > > > > + * Returns:
> > > > > > > > + * @len if successful, negative error code on failure.
> > > > > > > > + */
> > > > > > > > +int ttm_bo_access(struct ttm_buffer_object *bo, unsigned long offset,
> > > > > > > > + void *buf, int len, int write)
> > > > > > > > +{
> > > > > > > > + int ret;
> > > > > > > > +
> > > > > > > > + if (len < 1 || (offset + len) > bo->base.size)
> > > > > > > > + return -EIO;
> > > > > > > > +
> > > > > > > > + ret = ttm_bo_reserve(bo, true, false, NULL);
> > > > > > > > + if (ret)
> > > > > > > > + return ret;
> > > > > > > > +
> > > > > > > > + switch (bo->resource->mem_type) {
> > > > > > > > + case TTM_PL_SYSTEM:
> > > > > > > > + fallthrough;
> > > > > > > > + case TTM_PL_TT:
> > > > > > > > + ret = ttm_bo_access_kmap(bo, offset, buf, len, write);
> > > > > > > > + break;
> > > > > > > > + default:
> > > > > > > > + if (bo->bdev->funcs->access_memory)
> > > > > > > > + ret = bo->bdev->funcs->access_memory
> > > > > > > > + (bo, offset, buf, len, write);
> > > > > > > > + else
> > > > > > > > + ret = -EIO;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + ttm_bo_unreserve(bo);
> > > > > > > > +
> > > > > > > > + return ret;
> > > > > > > > +}
> > > > > > > > +EXPORT_SYMBOL(ttm_bo_access);
> > > > > > > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > > > index 2c699ed1963a..20b1e5f78684 100644
> > > > > > > > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > > > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > > > > > > > @@ -366,45 +366,6 @@ void ttm_bo_vm_close(struct vm_area_struct *vma)
> > > > > > > > }
> > > > > > > > EXPORT_SYMBOL(ttm_bo_vm_close);
> > > > > > > > -static int ttm_bo_vm_access_kmap(struct ttm_buffer_object *bo,
> > > > > > > > - unsigned long offset,
> > > > > > > > - uint8_t *buf, int len, int write)
> > > > > > > > -{
> > > > > > > > - unsigned long page = offset >> PAGE_SHIFT;
> > > > > > > > - unsigned long bytes_left = len;
> > > > > > > > - int ret;
> > > > > > > > -
> > > > > > > > - /* Copy a page at a time, that way no extra virtual address
> > > > > > > > - * mapping is needed
> > > > > > > > - */
> > > > > > > > - offset -= page << PAGE_SHIFT;
> > > > > > > > - do {
> > > > > > > > - unsigned long bytes = min(bytes_left, PAGE_SIZE - offset);
> > > > > > > > - struct ttm_bo_kmap_obj map;
> > > > > > > > - void *ptr;
> > > > > > > > - bool is_iomem;
> > > > > > > > -
> > > > > > > > - ret = ttm_bo_kmap(bo, page, 1, &map);
> > > > > > > > - if (ret)
> > > > > > > > - return ret;
> > > > > > > > -
> > > > > > > > - ptr = (uint8_t *)ttm_kmap_obj_virtual(&map, &is_iomem) + offset;
> > > > > > > > - WARN_ON_ONCE(is_iomem);
> > > > > > > > - if (write)
> > > > > > > > - memcpy(ptr, buf, bytes);
> > > > > > > > - else
> > > > > > > > - memcpy(buf, ptr, bytes);
> > > > > > > > - ttm_bo_kunmap(&map);
> > > > > > > > -
> > > > > > > > - page++;
> > > > > > > > - buf += bytes;
> > > > > > > > - bytes_left -= bytes;
> > > > > > > > - offset = 0;
> > > > > > > > - } while (bytes_left);
> > > > > > > > -
> > > > > > > > - return len;
> > > > > > > > -}
> > > > > > > > -
> > > > > > > > int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
> > > > > > > > void *buf, int len, int write)
> > > > > > > > {
> > > > > > > > @@ -412,32 +373,8 @@ int ttm_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
> > > > > > > > unsigned long offset = (addr) - vma->vm_start +
> > > > > > > > ((vma->vm_pgoff - drm_vma_node_start(&bo->base.vma_node))
> > > > > > > > << PAGE_SHIFT);
> > > > > > > > - int ret;
> > > > > > > > -
> > > > > > > > - if (len < 1 || (offset + len) > bo->base.size)
> > > > > > > > - return -EIO;
> > > > > > > > - ret = ttm_bo_reserve(bo, true, false, NULL);
> > > > > > > > - if (ret)
> > > > > > > > - return ret;
> > > > > > > > -
> > > > > > > > - switch (bo->resource->mem_type) {
> > > > > > > > - case TTM_PL_SYSTEM:
> > > > > > > > - fallthrough;
> > > > > > > > - case TTM_PL_TT:
> > > > > > > > - ret = ttm_bo_vm_access_kmap(bo, offset, buf, len, write);
> > > > > > > > - break;
> > > > > > > > - default:
> > > > > > > > - if (bo->bdev->funcs->access_memory)
> > > > > > > > - ret = bo->bdev->funcs->access_memory(
> > > > > > > > - bo, offset, buf, len, write);
> > > > > > > > - else
> > > > > > > > - ret = -EIO;
> > > > > > > > - }
> > > > > > > > -
> > > > > > > > - ttm_bo_unreserve(bo);
> > > > > > > > -
> > > > > > > > - return ret;
> > > > > > > > + return ttm_bo_access(bo, offset, buf, len, write);
> > > > > > > > }
> > > > > > > > EXPORT_SYMBOL(ttm_bo_vm_access);
> > > > > > > > diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
> > > > > > > > index 5804408815be..8ea11cd8df39 100644
> > > > > > > > --- a/include/drm/ttm/ttm_bo.h
> > > > > > > > +++ b/include/drm/ttm/ttm_bo.h
> > > > > > > > @@ -421,6 +421,8 @@ void ttm_bo_unpin(struct ttm_buffer_object *bo);
> > > > > > > > int ttm_bo_evict_first(struct ttm_device *bdev,
> > > > > > > > struct ttm_resource_manager *man,
> > > > > > > > struct ttm_operation_ctx *ctx);
> > > > > > > > +int ttm_bo_access(struct ttm_buffer_object *bo, unsigned long offset,
> > > > > > > > + void *buf, int len, int write);
> > > > > > > > vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
> > > > > > > > struct vm_fault *vmf);
> > > > > > > > vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
> > > > > > > > --
> > > > > > > > 2.34.1
> > > > > > > >
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