Commit 18a09e78 authored by Iago Toral's avatar Iago Toral

v3d: fix indirect BO allocation for uniforms

We were always ensuring a minimum size of 4 bytes for uniforms
for the case where we don't have any, to account for hardware pre-fetching
of the uniform stream, however, pre-fetching could also lead to to out
of bounds reads when have read the last uniform in the stream, so we
probably want to have the extra 4 bytes to prevent the kernel from
observing invalid memory accesses when the uniform stream sits right at
the end of a page.

This seems to fix MMU exceptions reported with a Linux 5.4 kernel.

Credit goes to Phil Elwell for identifying the problem and narrowing
it down to memory accesses in the uniform stream.
Reported-by: Phil Elwell's avatarPhil Elwell <>
Tested-by: Phil Elwell's avatarPhil Elwell <>
Reviewed-by: Eric Anholt's avatarEric Anholt <>
parent a24f1c8f
Pipeline #83175 passed with stages
in 29 minutes and 31 seconds
......@@ -215,10 +215,15 @@ v3d_write_uniforms(struct v3d_context *v3d, struct v3d_job *job,
struct v3d_uniform_list *uinfo = &shader->prog_data.base->uniforms;
const uint32_t *gallium_uniforms = cb->cb[0].user_buffer;
/* We always need to return some space for uniforms, because the HW
* will be prefetching, even if we don't read any in the program.
/* The hardware always pre-fetches the next uniform (also when there
* aren't any), so we always allocate space for an extra slot. This
* fixes MMU exceptions reported since Linux kernel 5.4 when the
* uniforms fill up the tail bytes of a page in the indirect
* BO. In that scenario, when the hardware pre-fetches after reading
* the last uniform it will read beyond the end of the page and trigger
* the MMU exception.
v3d_cl_ensure_space(&job->indirect, MAX2(uinfo->count, 1) * 4, 4);
v3d_cl_ensure_space(&job->indirect, (uinfo->count + 1) * 4, 4);
struct v3d_cl_reloc uniform_stream = cl_get_address(&job->indirect);
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