Re: [Intel-gfx] [PATCH 15/15] RFC: drm/amdgpu: Implement a proper implicit fencing uapi

Christian König
2021-06-23T14:02:45Z

Am 23.06.21 um 15:49 schrieb Daniel Vetter:
> On Wed, Jun 23, 2021 at 3:44 PM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 23.06.21 um 15:38 schrieb Bas Nieuwenhuizen:
>>> On Wed, Jun 23, 2021 at 2:59 PM Christian König
>>> <christian.koenig@amd.com> wrote:
>>>> Am 23.06.21 um 14:18 schrieb Daniel Vetter:
>>>>> On Wed, Jun 23, 2021 at 11:45 AM Bas Nieuwenhuizen
>>>>> <bas@basnieuwenhuizen.nl> wrote:
>>>>>> On Tue, Jun 22, 2021 at 6:55 PM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>>>>>>> WARNING: Absolutely untested beyond "gcc isn't dying in agony".
>>>>>>>
>>>>>>> Implicit fencing done properly needs to treat the implicit fencing
>>>>>>> slots like a funny kind of IPC mailbox. In other words it needs to be
>>>>>>> explicitly. This is the only way it will mesh well with explicit
>>>>>>> fencing userspace like vk, and it's also the bare minimum required to
>>>>>>> be able to manage anything else that wants to use the same buffer on
>>>>>>> multiple engines in parallel, and still be able to share it through
>>>>>>> implicit sync.
>>>>>>>
>>>>>>> amdgpu completely lacks such an uapi. Fix this.
>>>>>>>
>>>>>>> Luckily the concept of ignoring implicit fences exists already, and
>>>>>>> takes care of all the complexities of making sure that non-optional
>>>>>>> fences (like bo moves) are not ignored. This support was added in
>>>>>>>
>>>>>>> commit 177ae09b5d699a5ebd1cafcee78889db968abf54
>>>>>>> Author: Andres Rodriguez <andresx7@gmail.com>
>>>>>>> Date: Fri Sep 15 20:44:06 2017 -0400
>>>>>>>
>>>>>>> drm/amdgpu: introduce AMDGPU_GEM_CREATE_EXPLICIT_SYNC v2
>>>>>>>
>>>>>>> Unfortuantely it's the wrong semantics, because it's a bo flag and
>>>>>>> disables implicit sync on an allocated buffer completely.
>>>>>>>
>>>>>>> We _do_ want implicit sync, but control it explicitly. For this we
>>>>>>> need a flag on the drm_file, so that a given userspace (like vulkan)
>>>>>>> can manage the implicit sync slots explicitly. The other side of the
>>>>>>> pipeline (compositor, other process or just different stage in a media
>>>>>>> pipeline in the same process) can then either do the same, or fully
>>>>>>> participate in the implicit sync as implemented by the kernel by
>>>>>>> default.
>>>>>>>
>>>>>>> By building on the existing flag for buffers we avoid any issues with
>>>>>>> opening up additional security concerns - anything this new flag here
>>>>>>> allows is already.
>>>>>>>
>>>>>>> All drivers which supports this concept of a userspace-specific
>>>>>>> opt-out of implicit sync have a flag in their CS ioctl, but in reality
>>>>>>> that turned out to be a bit too inflexible. See the discussion below,
>>>>>>> let's try to do a bit better for amdgpu.
>>>>>>>
>>>>>>> This alone only allows us to completely avoid any stalls due to
>>>>>>> implicit sync, it does not yet allow us to use implicit sync as a
>>>>>>> strange form of IPC for sync_file.
>>>>>>>
>>>>>>> For that we need two more pieces:
>>>>>>>
>>>>>>> - a way to get the current implicit sync fences out of a buffer. Could
>>>>>>> be done in a driver ioctl, but everyone needs this, and generally a
>>>>>>> dma-buf is involved anyway to establish the sharing. So an ioctl on
>>>>>>> the dma-buf makes a ton more sense:
>>>>>>>
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20210520190007.534046-4-jason%40jlekstrand.net%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C83dbdd0a1eb8442cbf7108d9364db51e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637600529684040802%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=fbdwtutEj93anZp6Pshs277QoMTHZxIy0Yl54T95rCw%3D&amp;reserved=0
>>>>>>>
>>>>>>> Current drivers in upstream solves this by having the opt-out flag
>>>>>>> on their CS ioctl. This has the downside that very often the CS
>>>>>>> which must actually stall for the implicit fence is run a while
>>>>>>> after the implicit fence point was logically sampled per the api
>>>>>>> spec (vk passes an explicit syncobj around for that afaiui), and so
>>>>>>> results in oversync. Converting the implicit sync fences into a
>>>>>>> snap-shot sync_file is actually accurate.
>>>>>>>
>>>>>>> - Simillar we need to be able to set the exclusive implicit fence.
>>>>>>> Current drivers again do this with a CS ioctl flag, with again the
>>>>>>> same problems that the time the CS happens additional dependencies
>>>>>>> have been added. An explicit ioctl to only insert a sync_file (while
>>>>>>> respecting the rules for how exclusive and shared fence slots must
>>>>>>> be update in struct dma_resv) is much better. This is proposed here:
>>>>>>>
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20210520190007.534046-5-jason%40jlekstrand.net%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C83dbdd0a1eb8442cbf7108d9364db51e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637600529684040802%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=vv%2BREnWorjoTOwrD1jH1GHVQcjPy1oesaophsz056aI%3D&amp;reserved=0
>>>>>>>
>>>>>>> These three pieces together allow userspace to fully control implicit
>>>>>>> fencing and remove all unecessary stall points due to them.
>>>>>>>
>>>>>>> Well, as much as the implicit fencing model fundamentally allows:
>>>>>>> There is only one set of fences, you can only choose to sync against
>>>>>>> only writers (exclusive slot), or everyone. Hence suballocating
>>>>>>> multiple buffers or anything else like this is fundamentally not
>>>>>>> possible, and can only be fixed by a proper explicit fencing model.
>>>>>>>
>>>>>>> Aside from that caveat this model gets implicit fencing as closely to
>>>>>>> explicit fencing semantics as possible:
>>>>>>>
>>>>>>> On the actual implementation I opted for a simple setparam ioctl, no
>>>>>>> locking (just atomic reads/writes) for simplicity. There is a nice
>>>>>>> flag parameter in the VM ioctl which we could use, except:
>>>>>>> - it's not checked, so userspace likely passes garbage
>>>>>>> - there's already a comment that userspace _does_ pass garbage in the
>>>>>>> priority field
>>>>>>> So yeah unfortunately this flag parameter for setting vm flags is
>>>>>>> useless, and we need to hack up a new one.
>>>>>>>
>>>>>>> v2: Explain why a new SETPARAM (Jason)
>>>>>>>
>>>>>>> v3: Bas noticed I forgot to hook up the dependency-side shortcut. We
>>>>>>> need both, or this doesn't do much.
>>>>>>>
>>>>>>> v4: Rebase over the amdgpu patch to always set the implicit sync
>>>>>>> fences.
>>>>>> So I think there is still a case missing in this implementation.
>>>>>> Consider these 3 cases
>>>>>>
>>>>>> (format: a->b: b waits on a. Yes, I know arrows are hard)
>>>>>>
>>>>>> explicit->explicit: This doesn't wait now, which is good
>>>>>> Implicit->explicit: This doesn't wait now, which is good
>>>>>> explicit->implicit : This still waits as the explicit submission still
>>>>>> adds shared fences and most things that set an exclusive fence for
>>>>>> implicit sync will hence wait on it.
>>>>>>
>>>>>> This is probably good enough for what radv needs now but also sounds
>>>>>> like a risk wrt baking in new uapi behavior that we don't want to be
>>>>>> the end result.
>>>>>>
>>>>>> Within AMDGPU this is probably solvable in two ways:
>>>>>>
>>>>>> 1) Downgrade AMDGPU_SYNC_NE_OWNER to AMDGPU_SYNC_EXPLICIT for shared fences.
>>>>> I'm not sure that works. I think the right fix is that radeonsi also
>>>>> switches to this model, with maybe a per-bo CS flag to set indicate
>>>>> write access, to cut down on the number of ioctls that are needed
>>>>> otherwise on shared buffers. This per-bo flag would essentially select
>>>>> between SYNC_NE_OWNER and SYNC_EXPLICIT on a per-buffer basis.
>>>> Yeah, but I'm still not entirely sure why that approach isn't sufficient?
>>>>
>>>> Problem with the per context or per vm flag is that you then don't get
>>>> any implicit synchronization any more when another process starts using
>>>> the buffer.
>>> That is exactly what I want for Vulkan :)
>> Yeah, but as far as I know this is not something we can do.
>>
>> See we have use cases like screen capture and debug which rely on that
>> behavior.
> They will keep working, if (and only if) the vulkan side sets the
> winsys fences correctly. Also, everything else in vulkan aside from
> winsys is explicitly not synced at all, you have to import drm syncobj
> timeline on the gl side.
>
>> The only thing we can do is to say on a per buffer flag that a buffer
>> should not participate in implicit sync at all.
> Nah, this doesn't work. Because it's not a global decision, is a local
> decision for the rendered. Vulkan wants to control implicit sync
> explicitly, and the kernel can't force more synchronization. If a
> buffer is shared as a winsys buffer between vulkan client and gl using
> compositor, then you _have_ to use implicit sync on it. But vk needs
> to set the fences directly (and if the app gets it wrong, you get
> misrendering, but that is the specified behavour of vulkan).

Yeah, but that's exactly what we tried to avoid.

Mhm, when we attach the flag to the process/VM then this would break the
use case of VA-API and Vulkan in the same process.

But I think if you attach the flag to the context that should indeed
work fine.

Christian.

> -Daniel
>
>> Regards,
>> Christian.
>>
>>>>> The current amdgpu uapi just doesn't allow any other model without an
>>>>> explicit opt-in. So current implicit sync userspace just has to
>>>>> oversync, there's not much choice.
>>>>>
>>>>>> 2) Have an EXPLICIT fence owner that is used for explicit submissions
>>>>>> that is ignored by AMDGPU_SYNC_NE_OWNER.
>>>>>>
>>>>>> But this doesn't solve cross-driver interactions here.
>>>>> Yeah cross-driver is still entirely unsolved, because
>>>>> amdgpu_bo_explicit_sync() on the bo didn't solve that either.
>>>> Hui? You have lost me. Why is that still unsolved?
>>> The part we're trying to solve with this patch is Vulkan should not
>>> participate in any implicit sync at all wrt submissions (and then
>>> handle the implicit sync for WSI explicitly using the fence
>>> import/export stuff that Jason wrote). As long we add shared fences to
>>> the dma_resv we participate in implicit sync (at the level of an
>>> implicit sync read) still, at least from the perspective of later jobs
>>> waiting on these fences.
>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> -Daniel
>>>>>
>>>>>>> Cc: mesa-dev@lists.freedesktop.org
>>>>>>> Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
>>>>>>> Cc: Dave Airlie <airlied@gmail.com>
>>>>>>> Cc: Rob Clark <robdclark@chromium.org>
>>>>>>> Cc: Kristian H. Kristensen <hoegsberg@google.com>
>>>>>>> Cc: Michel Dänzer <michel@daenzer.net>
>>>>>>> Cc: Daniel Stone <daniels@collabora.com>
>>>>>>> Cc: Sumit Semwal <sumit.semwal@linaro.org>
>>>>>>> Cc: "Christian König" <christian.koenig@amd.com>
>>>>>>> Cc: Alex Deucher <alexander.deucher@amd.com>
>>>>>>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>>>>> Cc: Deepak R Varma <mh12gx2825@gmail.com>
>>>>>>> Cc: Chen Li <chenli@uniontech.com>
>>>>>>> Cc: Kevin Wang <kevin1.wang@amd.com>
>>>>>>> Cc: Dennis Li <Dennis.Li@amd.com>
>>>>>>> Cc: Luben Tuikov <luben.tuikov@amd.com>
>>>>>>> Cc: linaro-mm-sig@lists.linaro.org
>>>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>>>>> ---
>>>>>>> drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 7 +++++--
>>>>>>> drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 21 +++++++++++++++++++++
>>>>>>> drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h | 6 ++++++
>>>>>>> include/uapi/drm/amdgpu_drm.h | 10 ++++++++++
>>>>>>> 4 files changed, 42 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>>>>>> index 65df34c17264..c5386d13eb4a 100644
>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>>>>>> @@ -498,6 +498,7 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p,
>>>>>>> struct amdgpu_bo *gds;
>>>>>>> struct amdgpu_bo *gws;
>>>>>>> struct amdgpu_bo *oa;
>>>>>>> + bool no_implicit_sync = READ_ONCE(fpriv->vm.no_implicit_sync);
>>>>>>> int r;
>>>>>>>
>>>>>>> INIT_LIST_HEAD(&p->validated);
>>>>>>> @@ -577,7 +578,8 @@ static int amdgpu_cs_parser_bos(struct amdgpu_cs_parser *p,
>>>>>>>
>>>>>>> e->bo_va = amdgpu_vm_bo_find(vm, bo);
>>>>>>>
>>>>>>> - if (bo->tbo.base.dma_buf && !amdgpu_bo_explicit_sync(bo)) {
>>>>>>> + if (bo->tbo.base.dma_buf &&
>>>>>>> + !(no_implicit_sync || amdgpu_bo_explicit_sync(bo))) {
>>>>>>> e->chain = dma_fence_chain_alloc();
>>>>>>> if (!e->chain) {
>>>>>>> r = -ENOMEM;
>>>>>>> @@ -649,6 +651,7 @@ static int amdgpu_cs_sync_rings(struct amdgpu_cs_parser *p)
>>>>>>> {
>>>>>>> struct amdgpu_fpriv *fpriv = p->filp->driver_priv;
>>>>>>> struct amdgpu_bo_list_entry *e;
>>>>>>> + bool no_implicit_sync = READ_ONCE(fpriv->vm.no_implicit_sync);
>>>>>>> int r;
>>>>>>>
>>>>>>> list_for_each_entry(e, &p->validated, tv.head) {
>>>>>>> @@ -656,7 +659,7 @@ static int amdgpu_cs_sync_rings(struct amdgpu_cs_parser *p)
>>>>>>> struct dma_resv *resv = bo->tbo.base.resv;
>>>>>>> enum amdgpu_sync_mode sync_mode;
>>>>>>>
>>>>>>> - sync_mode = amdgpu_bo_explicit_sync(bo) ?
>>>>>>> + sync_mode = no_implicit_sync || amdgpu_bo_explicit_sync(bo) ?
>>>>>>> AMDGPU_SYNC_EXPLICIT : AMDGPU_SYNC_NE_OWNER;
>>>>>>> r = amdgpu_sync_resv(p->adev, &p->job->sync, resv, sync_mode,
>>>>>>> &fpriv->vm);
>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>> index c080ba15ae77..f982626b5328 100644
>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>> @@ -1724,6 +1724,26 @@ int amdgpu_file_to_fpriv(struct file *filp, struct amdgpu_fpriv **fpriv)
>>>>>>> return 0;
>>>>>>> }
>>>>>>>
>>>>>>> +int amdgpu_setparam_ioctl(struct drm_device *dev, void *data,
>>>>>>> + struct drm_file *filp)
>>>>>>> +{
>>>>>>> + struct drm_amdgpu_setparam *setparam = data;
>>>>>>> + struct amdgpu_fpriv *fpriv = filp->driver_priv;
>>>>>>> +
>>>>>>> + switch (setparam->param) {
>>>>>>> + case AMDGPU_SETPARAM_NO_IMPLICIT_SYNC:
>>>>>>> + if (setparam->value)
>>>>>>> + WRITE_ONCE(fpriv->vm.no_implicit_sync, true);
>>>>>>> + else
>>>>>>> + WRITE_ONCE(fpriv->vm.no_implicit_sync, false);
>>>>>>> + break;
>>>>>>> + default:
>>>>>>> + return -EINVAL;
>>>>>>> + }
>>>>>>> +
>>>>>>> + return 0;
>>>>>>> +}
>>>>>>> +
>>>>>>> const struct drm_ioctl_desc amdgpu_ioctls_kms[] = {
>>>>>>> DRM_IOCTL_DEF_DRV(AMDGPU_GEM_CREATE, amdgpu_gem_create_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> DRM_IOCTL_DEF_DRV(AMDGPU_CTX, amdgpu_ctx_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> @@ -1742,6 +1762,7 @@ const struct drm_ioctl_desc amdgpu_ioctls_kms[] = {
>>>>>>> DRM_IOCTL_DEF_DRV(AMDGPU_GEM_VA, amdgpu_gem_va_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> DRM_IOCTL_DEF_DRV(AMDGPU_GEM_OP, amdgpu_gem_op_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> DRM_IOCTL_DEF_DRV(AMDGPU_GEM_USERPTR, amdgpu_gem_userptr_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> + DRM_IOCTL_DEF_DRV(AMDGPU_SETPARAM, amdgpu_setparam_ioctl, DRM_AUTH|DRM_RENDER_ALLOW),
>>>>>>> };
>>>>>>>
>>>>>>> static const struct drm_driver amdgpu_kms_driver = {
>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
>>>>>>> index ddb85a85cbba..0e8c440c6303 100644
>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
>>>>>>> @@ -321,6 +321,12 @@ struct amdgpu_vm {
>>>>>>> bool bulk_moveable;
>>>>>>> /* Flag to indicate if VM is used for compute */
>>>>>>> bool is_compute_context;
>>>>>>> + /*
>>>>>>> + * Flag to indicate whether implicit sync should always be skipped on
>>>>>>> + * this context. We do not care about races at all, userspace is allowed
>>>>>>> + * to shoot itself with implicit sync to its fullest liking.
>>>>>>> + */
>>>>>>> + bool no_implicit_sync;
>>>>>>> };
>>>>>>>
>>>>>>> struct amdgpu_vm_manager {
>>>>>>> diff --git a/include/uapi/drm/amdgpu_drm.h b/include/uapi/drm/amdgpu_drm.h
>>>>>>> index 0cbd1540aeac..9eae245c14d6 100644
>>>>>>> --- a/include/uapi/drm/amdgpu_drm.h
>>>>>>> +++ b/include/uapi/drm/amdgpu_drm.h
>>>>>>> @@ -54,6 +54,7 @@ extern "C" {
>>>>>>> #define DRM_AMDGPU_VM 0x13
>>>>>>> #define DRM_AMDGPU_FENCE_TO_HANDLE 0x14
>>>>>>> #define DRM_AMDGPU_SCHED 0x15
>>>>>>> +#define DRM_AMDGPU_SETPARAM 0x16
>>>>>>>
>>>>>>> #define DRM_IOCTL_AMDGPU_GEM_CREATE DRM_IOWR(DRM_COMMAND_BASE + DRM_AMDGPU_GEM_CREATE, union drm_amdgpu_gem_create)
>>>>>>> #define DRM_IOCTL_AMDGPU_GEM_MMAP DRM_IOWR(DRM_COMMAND_BASE + DRM_AMDGPU_GEM_MMAP, union drm_amdgpu_gem_mmap)
>>>>>>> @@ -71,6 +72,7 @@ extern "C" {
>>>>>>> #define DRM_IOCTL_AMDGPU_VM DRM_IOWR(DRM_COMMAND_BASE + DRM_AMDGPU_VM, union drm_amdgpu_vm)
>>>>>>> #define DRM_IOCTL_AMDGPU_FENCE_TO_HANDLE DRM_IOWR(DRM_COMMAND_BASE + DRM_AMDGPU_FENCE_TO_HANDLE, union drm_amdgpu_fence_to_handle)
>>>>>>> #define DRM_IOCTL_AMDGPU_SCHED DRM_IOW(DRM_COMMAND_BASE + DRM_AMDGPU_SCHED, union drm_amdgpu_sched)
>>>>>>> +#define DRM_IOCTL_AMDGPU_SETPARAM DRM_IOW(DRM_COMMAND_BASE + DRM_AMDGPU_SETPARAM, struct drm_amdgpu_setparam)
>>>>>>>
>>>>>>> /**
>>>>>>> * DOC: memory domains
>>>>>>> @@ -306,6 +308,14 @@ union drm_amdgpu_sched {
>>>>>>> struct drm_amdgpu_sched_in in;
>>>>>>> };
>>>>>>>
>>>>>>> +#define AMDGPU_SETPARAM_NO_IMPLICIT_SYNC 1
>>>>>>> +
>>>>>>> +struct drm_amdgpu_setparam {
>>>>>>> + /* AMDGPU_SETPARAM_* */
>>>>>>> + __u32 param;
>>>>>>> + __u32 value;
>>>>>>> +};
>>>>>>> +
>>>>>>> /*
>>>>>>> * This is not a reliable API and you should expect it to fail for any
>>>>>>> * number of reasons and have fallback path that do not use userptr to
>>>>>>> --
>>>>>>> 2.32.0.rc2
>>>>>>>