On 03.06.2021 07:16, Matthew Brost wrote:
> Ensure H2G buffer updates are visible before descriptor tail updates by
> inserting a barrier between the H2G buffer update and the tail. The
> barrier is simple wmb() for SMEM and is register write for LMEM. This is
> needed if more than 1 H2G can be inflight at once.
>
> If this barrier is not inserted it is possible the descriptor tail
> update is scene by the GuC before H2G buffer update which results in the
> GuC reading a corrupt H2G value. This can bring down the H2G channel
> among other bad things.
>
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx>
> Reviewed-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 28 +++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> index 80976fe40fbf..31f83956bfc3 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> @@ -328,6 +328,28 @@ static u32 ct_get_next_fence(struct intel_guc_ct *ct)
> return ++ct->requests.last_fence;
> }
>
> +static void write_barrier(struct intel_guc_ct *ct)
> +{
> + struct intel_guc *guc = ct_to_guc(ct);
> + struct intel_gt *gt = guc_to_gt(guc);
> +
> + if (i915_gem_object_is_lmem(guc->ct.vma->obj)) {
> + GEM_BUG_ON(guc->send_regs.fw_domains);
> + /*
> + * This register is used by the i915 and GuC for MMIO based
> + * communication. Once we are in this code CTBs are the only
> + * method the i915 uses to communicate with the GuC so it is
> + * safe to write to this register (a value of 0 is NOP for MMIO
> + * communication). If we ever start mixing CTBs and MMIOs a new
> + * register will have to be chosen.
> + */
> + intel_uncore_write_fw(gt->uncore, GEN11_SOFT_SCRATCH(0), 0);
can't we at least start with SOFT_SCRATCH register that is not used for
GuC MMIO based communication on Gen12 LMEM platforms? see [1]
I really don't feel comfortable that we are touching a register that
elsewhere is protected with the mutex. And mixing CTBs and MMIO is not
far away.
Michal
[1]
https://lore.kernel.org/intel-gfx/51b9bd05-7d6f-29f1-de0f-3a14bade6c9c@xxxxxxxxx/
> + } else {
> + /* wmb() sufficient for a barrier if in smem */
> + wmb();
> + }
> +}
> +
> /**
> * DOC: CTB Host to GuC request
> *
> @@ -411,6 +433,12 @@ static int ct_write(struct intel_guc_ct *ct,
> }
> GEM_BUG_ON(tail > size);
>
> + /*
> + * make sure H2G buffer update and LRC tail update (if this triggering a
> + * submission) are visible before updating the descriptor tail
> + */
> + write_barrier(ct);
> +
> /* now update desc tail (back in bytes) */
> desc->tail = tail * 4;
> return 0;
>
