Re: [Intel-gfx] [PATCH 15/20] drm/i915/guc: Ensure H2G buffer updates visible before tail update

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

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@intel.com>
> Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
> Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
> ---
>  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@intel.com/

> +	} 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;
> 
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