[PATCH] drm/msm/a6xx: Track current ctx by seqno

[PATCH] drm/msm/a6xx: Track current ctx by seqno

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

In theory a context can be destroyed and a new one allocated at the same
address, making the pointer comparision to detect when we don't need to
update the current pagetables invalid.  Instead assign a sequence number
to each context on creation, and use this for the check.

Fixes: 84c31ee16f90 ("drm/msm/a6xx: Add support for per-instance pagetables")
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/msm/adreno/a6xx_gpu.c |  6 +++---
 drivers/gpu/drm/msm/adreno/a6xx_gpu.h | 11 ++++++++++-
 drivers/gpu/drm/msm/msm_drv.c         |  3 +++
 drivers/gpu/drm/msm/msm_drv.h         |  1 +
 4 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
index 5e1ae3df42ba..e0a8b2fd1ff0 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
@@ -106,7 +106,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
 	u32 asid;
 	u64 memptr = rbmemptr(ring, ttbr0);
 
-	if (ctx == a6xx_gpu->cur_ctx)
+	if (ctx->seqno == a6xx_gpu->cur_ctx_seqno)
 		return;
 
 	if (msm_iommu_pagetable_params(ctx->aspace->mmu, &ttbr, &asid))
@@ -139,7 +139,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
 	OUT_PKT7(ring, CP_EVENT_WRITE, 1);
 	OUT_RING(ring, 0x31);
 
-	a6xx_gpu->cur_ctx = ctx;
+	a6xx_gpu->cur_ctx_seqno = ctx->seqno;
 }
 
 static void a6xx_submit(struct msm_gpu *gpu, struct msm_gem_submit *submit)
@@ -1081,7 +1081,7 @@ static int hw_init(struct msm_gpu *gpu)
 	/* Always come up on rb 0 */
 	a6xx_gpu->cur_ring = gpu->rb[0];
 
-	a6xx_gpu->cur_ctx = NULL;
+	a6xx_gpu->cur_ctx_seqno = 0;
 
 	/* Enable the SQE_to start the CP engine */
 	gpu_write(gpu, REG_A6XX_CP_SQE_CNTL, 1);
diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
index 0bc2d062f54a..8e5527c881b1 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
@@ -19,7 +19,16 @@ struct a6xx_gpu {
 	uint64_t sqe_iova;
 
 	struct msm_ringbuffer *cur_ring;
-	struct msm_file_private *cur_ctx;
+
+	/**
+	 * cur_ctx_seqno:
+	 *
+	 * The ctx->seqno value of the context with current pgtables
+	 * installed.  Tracked by seqno rather than pointer value to
+	 * avoid dangling pointers, and cases where a ctx can be freed
+	 * and a new one created with the same address.
+	 */
+	int cur_ctx_seqno;
 
 	struct a6xx_gmu gmu;
 
diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
index 624078b3adf2..30c1efc3d8a0 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -711,6 +711,7 @@ static void load_gpu(struct drm_device *dev)
 
 static int context_init(struct drm_device *dev, struct drm_file *file)
 {
+	static atomic_t ident = ATOMIC_INIT(0);
 	struct msm_drm_private *priv = dev->dev_private;
 	struct msm_file_private *ctx;
 
@@ -727,6 +728,8 @@ static int context_init(struct drm_device *dev, struct drm_file *file)
 	ctx->aspace = msm_gpu_create_private_address_space(priv->gpu, current);
 	file->driver_priv = ctx;
 
+	ctx->seqno = atomic_inc_return(&ident);
+
 	return 0;
 }
 
diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
index de062450add4..8633d0059a3e 100644
--- a/drivers/gpu/drm/msm/msm_drv.h
+++ b/drivers/gpu/drm/msm/msm_drv.h
@@ -59,6 +59,7 @@ struct msm_file_private {
 	int queueid;
 	struct msm_gem_address_space *aspace;
 	struct kref ref;
+	int seqno;
 };
 
 enum msm_mdp_plane_property {
-- 
2.31.1

Re: [PATCH] drm/msm/a6xx: Track current ctx by seqno

[Daniel Vetter]

On Thu, Sep 30, 2021 at 8:20 PM Rob Clark <robdclark@gmail.com> wrote:
>
> From: Rob Clark <robdclark@chromium.org>
>
> In theory a context can be destroyed and a new one allocated at the same
> address, making the pointer comparision to detect when we don't need to
> update the current pagetables invalid.  Instead assign a sequence number
> to each context on creation, and use this for the check.
>
> Fixes: 84c31ee16f90 ("drm/msm/a6xx: Add support for per-instance pagetables")
> Signed-off-by: Rob Clark <robdclark@chromium.org>

Usually the weak reference pattern is to wrap a spinlock or something
around it and clear it out on destruction. Or hold a full reference
(e.g. on intel hw the hw goes unhappy if there's no context/pagetables
at all, so we always need some). But I guess this works too.
-Daniel

> ---
>  drivers/gpu/drm/msm/adreno/a6xx_gpu.c |  6 +++---
>  drivers/gpu/drm/msm/adreno/a6xx_gpu.h | 11 ++++++++++-
>  drivers/gpu/drm/msm/msm_drv.c         |  3 +++
>  drivers/gpu/drm/msm/msm_drv.h         |  1 +
>  4 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> index 5e1ae3df42ba..e0a8b2fd1ff0 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> @@ -106,7 +106,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
>         u32 asid;
>         u64 memptr = rbmemptr(ring, ttbr0);
>
> -       if (ctx == a6xx_gpu->cur_ctx)
> +       if (ctx->seqno == a6xx_gpu->cur_ctx_seqno)
>                 return;
>
>         if (msm_iommu_pagetable_params(ctx->aspace->mmu, &ttbr, &asid))
> @@ -139,7 +139,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
>         OUT_PKT7(ring, CP_EVENT_WRITE, 1);
>         OUT_RING(ring, 0x31);
>
> -       a6xx_gpu->cur_ctx = ctx;
> +       a6xx_gpu->cur_ctx_seqno = ctx->seqno;
>  }
>
>  static void a6xx_submit(struct msm_gpu *gpu, struct msm_gem_submit *submit)
> @@ -1081,7 +1081,7 @@ static int hw_init(struct msm_gpu *gpu)
>         /* Always come up on rb 0 */
>         a6xx_gpu->cur_ring = gpu->rb[0];
>
> -       a6xx_gpu->cur_ctx = NULL;
> +       a6xx_gpu->cur_ctx_seqno = 0;
>
>         /* Enable the SQE_to start the CP engine */
>         gpu_write(gpu, REG_A6XX_CP_SQE_CNTL, 1);
> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> index 0bc2d062f54a..8e5527c881b1 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> @@ -19,7 +19,16 @@ struct a6xx_gpu {
>         uint64_t sqe_iova;
>
>         struct msm_ringbuffer *cur_ring;
> -       struct msm_file_private *cur_ctx;
> +
> +       /**
> +        * cur_ctx_seqno:
> +        *
> +        * The ctx->seqno value of the context with current pgtables
> +        * installed.  Tracked by seqno rather than pointer value to
> +        * avoid dangling pointers, and cases where a ctx can be freed
> +        * and a new one created with the same address.
> +        */
> +       int cur_ctx_seqno;
>
>         struct a6xx_gmu gmu;
>
> diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> index 624078b3adf2..30c1efc3d8a0 100644
> --- a/drivers/gpu/drm/msm/msm_drv.c
> +++ b/drivers/gpu/drm/msm/msm_drv.c
> @@ -711,6 +711,7 @@ static void load_gpu(struct drm_device *dev)
>
>  static int context_init(struct drm_device *dev, struct drm_file *file)
>  {
> +       static atomic_t ident = ATOMIC_INIT(0);
>         struct msm_drm_private *priv = dev->dev_private;
>         struct msm_file_private *ctx;
>
> @@ -727,6 +728,8 @@ static int context_init(struct drm_device *dev, struct drm_file *file)
>         ctx->aspace = msm_gpu_create_private_address_space(priv->gpu, current);
>         file->driver_priv = ctx;
>
> +       ctx->seqno = atomic_inc_return(&ident);
> +
>         return 0;
>  }
>
> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> index de062450add4..8633d0059a3e 100644
> --- a/drivers/gpu/drm/msm/msm_drv.h
> +++ b/drivers/gpu/drm/msm/msm_drv.h
> @@ -59,6 +59,7 @@ struct msm_file_private {
>         int queueid;
>         struct msm_gem_address_space *aspace;
>         struct kref ref;
> +       int seqno;
>  };
>
>  enum msm_mdp_plane_property {
> --
> 2.31.1
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm/msm/a6xx: Track current ctx by seqno

[Rob Clark]

On Thu, Sep 30, 2021 at 11:34 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Thu, Sep 30, 2021 at 8:20 PM Rob Clark <robdclark@gmail.com> wrote:
> >
> > From: Rob Clark <robdclark@chromium.org>
> >
> > In theory a context can be destroyed and a new one allocated at the same
> > address, making the pointer comparision to detect when we don't need to
> > update the current pagetables invalid.  Instead assign a sequence number
> > to each context on creation, and use this for the check.
> >
> > Fixes: 84c31ee16f90 ("drm/msm/a6xx: Add support for per-instance pagetables")
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
>
> Usually the weak reference pattern is to wrap a spinlock or something
> around it and clear it out on destruction. Or hold a full reference
> (e.g. on intel hw the hw goes unhappy if there's no context/pagetables
> at all, so we always need some). But I guess this works too.

yeah, the seqno approach was so that gen agnostic code didn't have to
go fishing around in a6xx_gpu..

I guess the downside is userspace could open/close the dev file 2^32
times without submitting any rendering to trigger the same sort of
issue.  But there are plenty of easier ways for userspace to trigger
faults if it wanted.

BR,
-R

> -Daniel
>
> > ---
> >  drivers/gpu/drm/msm/adreno/a6xx_gpu.c |  6 +++---
> >  drivers/gpu/drm/msm/adreno/a6xx_gpu.h | 11 ++++++++++-
> >  drivers/gpu/drm/msm/msm_drv.c         |  3 +++
> >  drivers/gpu/drm/msm/msm_drv.h         |  1 +
> >  4 files changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > index 5e1ae3df42ba..e0a8b2fd1ff0 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > @@ -106,7 +106,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
> >         u32 asid;
> >         u64 memptr = rbmemptr(ring, ttbr0);
> >
> > -       if (ctx == a6xx_gpu->cur_ctx)
> > +       if (ctx->seqno == a6xx_gpu->cur_ctx_seqno)
> >                 return;
> >
> >         if (msm_iommu_pagetable_params(ctx->aspace->mmu, &ttbr, &asid))
> > @@ -139,7 +139,7 @@ static void a6xx_set_pagetable(struct a6xx_gpu *a6xx_gpu,
> >         OUT_PKT7(ring, CP_EVENT_WRITE, 1);
> >         OUT_RING(ring, 0x31);
> >
> > -       a6xx_gpu->cur_ctx = ctx;
> > +       a6xx_gpu->cur_ctx_seqno = ctx->seqno;
> >  }
> >
> >  static void a6xx_submit(struct msm_gpu *gpu, struct msm_gem_submit *submit)
> > @@ -1081,7 +1081,7 @@ static int hw_init(struct msm_gpu *gpu)
> >         /* Always come up on rb 0 */
> >         a6xx_gpu->cur_ring = gpu->rb[0];
> >
> > -       a6xx_gpu->cur_ctx = NULL;
> > +       a6xx_gpu->cur_ctx_seqno = 0;
> >
> >         /* Enable the SQE_to start the CP engine */
> >         gpu_write(gpu, REG_A6XX_CP_SQE_CNTL, 1);
> > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > index 0bc2d062f54a..8e5527c881b1 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.h
> > @@ -19,7 +19,16 @@ struct a6xx_gpu {
> >         uint64_t sqe_iova;
> >
> >         struct msm_ringbuffer *cur_ring;
> > -       struct msm_file_private *cur_ctx;
> > +
> > +       /**
> > +        * cur_ctx_seqno:
> > +        *
> > +        * The ctx->seqno value of the context with current pgtables
> > +        * installed.  Tracked by seqno rather than pointer value to
> > +        * avoid dangling pointers, and cases where a ctx can be freed
> > +        * and a new one created with the same address.
> > +        */
> > +       int cur_ctx_seqno;
> >
> >         struct a6xx_gmu gmu;
> >
> > diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> > index 624078b3adf2..30c1efc3d8a0 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.c
> > +++ b/drivers/gpu/drm/msm/msm_drv.c
> > @@ -711,6 +711,7 @@ static void load_gpu(struct drm_device *dev)
> >
> >  static int context_init(struct drm_device *dev, struct drm_file *file)
> >  {
> > +       static atomic_t ident = ATOMIC_INIT(0);
> >         struct msm_drm_private *priv = dev->dev_private;
> >         struct msm_file_private *ctx;
> >
> > @@ -727,6 +728,8 @@ static int context_init(struct drm_device *dev, struct drm_file *file)
> >         ctx->aspace = msm_gpu_create_private_address_space(priv->gpu, current);
> >         file->driver_priv = ctx;
> >
> > +       ctx->seqno = atomic_inc_return(&ident);
> > +
> >         return 0;
> >  }
> >
> > diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> > index de062450add4..8633d0059a3e 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.h
> > +++ b/drivers/gpu/drm/msm/msm_drv.h
> > @@ -59,6 +59,7 @@ struct msm_file_private {
> >         int queueid;
> >         struct msm_gem_address_space *aspace;
> >         struct kref ref;
> > +       int seqno;
> >  };
> >
> >  enum msm_mdp_plane_property {
> > --
> > 2.31.1
> >
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch