Am 03.06.22 um 14:39 schrieb Bas Nieuwenhuizen:
On Fri, Jun 3, 2022 at 2:08 PM Christian König <christian.koenig@xxxxxxx> wrote:
Am 03.06.22 um 13:07 schrieb Bas Nieuwenhuizen:
On Fri, Jun 3, 2022 at 12:16 PM Christian König
<christian.koenig@xxxxxxx> wrote:
Am 03.06.22 um 12:08 schrieb Bas Nieuwenhuizen:
[SNIP]
I do have to fix some stuff indeed, especially for the GEM close but
with that we should be able to keep the same basic approach?
Nope, not even remotely.
What we need is the following:
1. Rolling out my drm_exec patch set, so that we can lock buffers as needed.
2. When we get a VM operation we not only lock the VM page tables, but
also all buffers we potentially need to unmap.
3. Nuking the freed list in the amdgpu_vm structure by updating freed
areas directly when they are unmapped.
4. Tracking those updates inside the bo_va structure for the BO+VM
combination.
5. When the bo_va structure is destroy because of closing the handle
move the last clear operation over to the VM as implicit sync.
Hi Christian, isn't that a different problem though (that we're also
trying to solve, but in your series)?
What this patch tries to achieve:
(t+0) CS submission setting BOOKKEEP fences (i.e. no implicit sync)
(t+1) a VM operation on a BO/VM accessed by the CS.
to run concurrently. What it *doesn't* try is
(t+0) a VM operation on a BO/VM accessed by the CS.
(t+1) CS submission setting BOOKKEEP fences (i.e. no implicit sync)
to run concurrently. When you write
Only when all this is done we then can resolve the dependency that the
CS currently must wait for any clear operation on the VM.
isn't that all about the second problem?
No, it's the same.
See what we do in the VM code is to artificially insert a bubble so that
all VM clear operations wait for all CS operations and then use the
clear fence to indicate when the backing store of the BO can be freed.
Isn't that remediated with something like the code below? At least the
gem_close case should be handled with this, and the move case was
already handled by the copy operation.
That is one necessary puzzle piece, yes. But you need more than that.
Especially the explicit unmap operation needs to be converted into an
implicit unmap to get the TLB flush right.
This doesn't change anything about the TLB flush though? Since all
unmap -> later jobs dependencies are still implicit.
So the worst what could happen (i.f. e.g. userspace gets the
waits/dependencies wrong) is
1) non-implicit CS gets submitted that touches a BO
2) VM unmap on that BO happens
2.5) the CS from 1 is still active due to missing dependencies
2.6) but any CS submission after 2 will trigger a TLB flush
Yeah, but that's exactly the bubble we try to avoid. Isn't it?
When we want to do a TLB flush the unmap operation must already be
completed. Otherwise the flush is rather pointless since any access
could reloads the not yet updated PTEs.
And this means that we need to artificially add a dependency on every
command submission after 2 to wait until the unmap operation is completed.
Christian.
3) A TLB flush happens for a new CS
4) All CS submissions here see the TLB flush and hence the unmap
So the main problem would be the CS from step 1, but (a) if that
VMFaults that is the apps own fault and (b) because we don't free the
memory until (1) finishes it is not a security issue kernel-wise.
I think I know all the necessary steps now, it's just tons of work to do.
Regards,
Christian.
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -187,6 +187,39 @@ static int amdgpu_gem_object_open(struct
drm_gem_object *obj,
return 0;
}
+static void dma_resv_copy(struct dma_resv *src, struct dma_resv *dst)
+{
+ struct dma_resv_iter cursor;
+ struct dma_fence *f;
+ int r;
+ unsigned num_fences = 0;
+
+ if (src == dst)
+ return;
+
+ /* We assume the later loops get the same fences as the caller should
+ * lock the resv. */
+ dma_resv_for_each_fence(&cursor, src, DMA_RESV_USAGE_BOOKKEEP, f) {
+ ++num_fences;
+ dma_fence_put(f);
+ }
+
+ r = dma_resv_reserve_fences(dst, num_fences);
+ if (r) {
+ /* As last resort on OOM we block for the fence */
+ dma_resv_for_each_fence(&cursor, src,
DMA_RESV_USAGE_BOOKKEEP, f) {
+ dma_fence_wait(f, false);
+ dma_fence_put(f);
+ }
+ }
+
+ dma_resv_for_each_fence(&cursor, src, DMA_RESV_USAGE_BOOKKEEP, f) {
+ dma_resv_add_fence(dst, f, dma_resv_iter_usage(&cursor));
+ dma_fence_put(f);
+ }
+}
+
+
static void amdgpu_gem_object_close(struct drm_gem_object *obj,
struct drm_file *file_priv)
{
@@ -233,6 +266,8 @@ static void amdgpu_gem_object_close(struct
drm_gem_object *obj,
amdgpu_bo_fence(bo, fence, true);
dma_fence_put(fence);
+ dma_resv_copy(vm->root.bo->tbo.base.resv, bo->tbo.base.resv);
+
out_unlock:
if (unlikely(r < 0))
dev_err(adev->dev, "failed to clear page "
When you want to remove this bubble (which is certainly a good idea) you
need to first come up with a different approach to handle the clear
operations.
Regards,
Christian.
Regards,
Christian.
