[Mesa-dev] [PATCH] anv: Don't place scratch buffers above the 32-bit boundary

[Kenneth Graunke]

On Saturday, April 22, 2017 3:55:07 PM PDT Jason Ekstrand wrote:
> This fixes rendering corruptions in DOOM.  Hopefully, it will also make
> Jenkins a bit more stable as we've been seeing some random failures and
> GPU hangs ever since turning on 48bit.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100620
> Cc: "17.1" <mesa-stable at lists.freedesktop.org>
> ---
>  src/intel/vulkan/anv_allocator.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/src/intel/vulkan/anv_allocator.c b/src/intel/vulkan/anv_allocator.c
> index 78327df..28bfac4 100644
> --- a/src/intel/vulkan/anv_allocator.c
> +++ b/src/intel/vulkan/anv_allocator.c
> @@ -994,6 +994,22 @@ anv_scratch_pool_alloc(struct anv_device *device, struct anv_scratch_pool *pool,
>  
>     anv_bo_init_new(&bo->bo, device, size);
>  
> +   /* Even though the Scratch base pointers in 3DSTATE_*S are 64 bits, they
> +    * are still relative to the general state base address.  When we emit
> +    * STATE_BASE_ADDRESS, we set general state base address to 0 and the size
> +    * to the maximum (1 page under 4GB).  This allows us to just place the
> +    * scratch buffers anywhere we wish in the bottom 32 bits of address space
> +    * and just set the scratch base pointer in 3DSTATE_*S using a relocation.
> +    * However, in order to do so, we need to ensure that the kernel does not
> +    * place the scratch BO above the 32-bit boundary.
> +    *
> +    * NOTE: Technically, it can't go "anywhere" because the top page is off
> +    * limits.  However, it will never end up getting placed that high because
> +    * the surface state and general state buffers will get placed first and
> +    * the kernel likes to work top-down.

This NOTE sounds unnecessarily scary.

If you don't set EXEC_OBJECT_SUPPORTS_48B_ADDRESS, then the kernel sets
this to PIN_ZONE_4G...which in i915_vma.c does:

                end = min_t(u64, end, (1ULL << 32) - I915_GTT_PAGE_SIZE);

If I'm reading the kernel code correctly, that means the kernel is
explicitly refusing to position it in that top page.  So it's not a
side effect of the kernel "liking to work top-down" - I'm pretty sure
it works by design, not by accident.

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

> +    */
> +   bo->bo.flags &= ~EXEC_OBJECT_SUPPORTS_48B_ADDRESS;
> +
>     /* Set the exists last because it may be read by other threads */
>     __sync_synchronize();
>     bo->exists = true;
> 
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