Re: [PATCH v6 2/8] drm/ttm: Add ttm_bo_access

[Matthew Brost <matthew.brost@intel.com>]

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@intel.com>
> > > > > > > > Suggested-by: Thomas Hellström<thomas.hellstrom@linux.intel.com>
> > > > > > > > Signed-off-by: Matthew Brost<matthew.brost@intel.com>
> > > > > > > > Tested-by: Mika Kuoppala<mika.kuoppala@linux.intel.com>
> > > > > > > > Reviewed-by: Matthew Auld<matthew.auld@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
> > > > > > > >