Re: [PATCH v4 2/2] drm/xe/guc: Enable the Dynamic Inhibit Context Switch optimization

[John Harrison <john.c.harrison@intel.com>]

On 6/3/2025 5:24 PM, Daniele Ceraolo Spurio wrote:
> The Dynamic Inhibit Context Switch is an optimization aimed at reducing
> the amount of time the HW is stuck waiting on an unsatisfied semaphore.
> When this optimization is enabled, the GuC will dynamically modify the
> CTX_CTRL_INHIBIT_SYN_CTX_SWITCH in the CTX_CONTEXT_CONTROL register of
> LRCs to enable immediate switching out on an unsatisfied semaphore wait
> when multiple contexts are competing for time on the same engine.
>
> This feature is available on recent HW from GuC 70.40.1 onwards and it
> is enabled via a per-VF feature opt-in.
>
> v2: rebase
> v3: switch to using guc_buf_cache instead of dedicated alloc
> v4: add helper to check for feature availability (Michal), don't enable
> if multi-lrc is possible.
>
> Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> Cc: John Harrison <John.C.Harrison@Intel.com>
> Cc: Julia Filipchuk <julia.filipchuk@intel.com>
> Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>

> ---
>   drivers/gpu/drm/xe/abi/guc_klvs_abi.h | 13 ++++++++++++
>   drivers/gpu/drm/xe/xe_guc.c           | 30 +++++++++++++++++++++++++++
>   2 files changed, 43 insertions(+)
>
> diff --git a/drivers/gpu/drm/xe/abi/guc_klvs_abi.h b/drivers/gpu/drm/xe/abi/guc_klvs_abi.h
> index 5b2502bec2dc..0366a9da5977 100644
> --- a/drivers/gpu/drm/xe/abi/guc_klvs_abi.h
> +++ b/drivers/gpu/drm/xe/abi/guc_klvs_abi.h
> @@ -134,11 +134,24 @@ enum  {
>    *      Adds an extra dword to the XE_GUC_ACTION_NOTIFY_MEMORY_CAT_ERROR G2H
>    *      containing the type of the CAT error. On HW that does not support
>    *      reporting the CAT error type, the extra dword is set to 0xdeadbeef.
> + *
> + * _`GUC_KLV_OPT_IN_FEATURE_DYNAMIC_INHIBIT_CONTEXT_SWITCH` : 0x4003
> + *      This KLV enables the Dynamic Inhibit Context Switch optimization, which
> + *      consists in the GuC setting the CTX_CTRL_INHIBIT_SYN_CTX_SWITCH bit to
> + *      zero in the CTX_CONTEXT_CONTROL register of LRCs that are submitted
> + *      to an oversubscribed engine. This will cause those contexts to be
> + *      switched out immediately if they hit an unsatisfied semaphore wait
> + *      (instead of waiting the full timeslice duration). The bit is instead set
> + *      to one if a single context is queued on the engine, to avoid it being
> + *      switched out if there isn't another context that can run in its place.
>    */
>   
>   #define GUC_KLV_OPT_IN_FEATURE_EXT_CAT_ERR_TYPE_KEY 0x4001
>   #define GUC_KLV_OPT_IN_FEATURE_EXT_CAT_ERR_TYPE_LEN 0u
>   
> +#define GUC_KLV_OPT_IN_FEATURE_DYNAMIC_INHIBIT_CONTEXT_SWITCH_KEY 0x4003
> +#define GUC_KLV_OPT_IN_FEATURE_DYNAMIC_INHIBIT_CONTEXT_SWITCH_LEN 0u
> +
>   /**
>    * DOC: GuC VGT Policy KLVs
>    *
> diff --git a/drivers/gpu/drm/xe/xe_guc.c b/drivers/gpu/drm/xe/xe_guc.c
> index 46419861bfcd..b94e7bd865d5 100644
> --- a/drivers/gpu/drm/xe/xe_guc.c
> +++ b/drivers/gpu/drm/xe/xe_guc.c
> @@ -583,6 +583,32 @@ static int __guc_opt_in_features_enable(struct xe_guc *guc, u64 addr, u32 num_dw
>   	return xe_guc_ct_send_block(&guc->ct, action, ARRAY_SIZE(action));
>   }
>   
> +static bool supports_dynamic_ics(struct xe_guc *guc)
> +{
> +	struct xe_device *xe = guc_to_xe(guc);
> +	struct xe_gt *gt = guc_to_gt(guc);
> +
> +	/* Dynamic ICS is available for PVC and Xe2 and newer platforms. */
> +	if (xe->info.platform != XE_PVC && GRAPHICS_VER(xe) < 20)
> +		return false;
> +
> +	/*
> +	 * The feature is currently not compatible with multi-lrc, so the GuC
> +	 * does not support it at all on the media engines (which are the main
> +	 * users of mlrc). On the primary GT side, to avoid it being used in
> +	 * conjunction with mlrc, we only enable it if we are in single CCS
> +	 * mode.
> +	 */
> +	if (xe_gt_is_media_type(gt) || gt->ccs_mode > 1)
> +		return false;
> +
> +	/*
> +	 * Dynamic ICS requires GuC v70.40.1, which maps to compatibility
> +	 * version v1.18.4.
> +	 */
> +	return GUC_SUBMIT_VER(guc) >= MAKE_GUC_VER(1, 18, 4);
> +}
> +
>   #define OPT_IN_MAX_DWORDS 16
>   int xe_guc_opt_in_features_enable(struct xe_guc *guc)
>   {
> @@ -607,6 +633,10 @@ int xe_guc_opt_in_features_enable(struct xe_guc *guc)
>   	if (GUC_SUBMIT_VER(guc) >= MAKE_GUC_VER(1, 7, 0))
>   		klvs[count++] = PREP_GUC_KLV_TAG(OPT_IN_FEATURE_EXT_CAT_ERR_TYPE);
>   
> +
> +	if (supports_dynamic_ics(guc))
> +		klvs[count++] = PREP_GUC_KLV_TAG(OPT_IN_FEATURE_DYNAMIC_INHIBIT_CONTEXT_SWITCH);
> +
>   	if (count) {
>   		xe_assert(xe, count <= OPT_IN_MAX_DWORDS);
>