[Intel-gfx] [PATCH v2 4/5] drm/i915/execlists: Read the context-status buffer from the HWSP

[Wang, Zhi A]

Sorry, my bad for missing it as I only saw it used on CHV before your patch because of some HW issue.

Kinda of interested in the reason why HWSP CSB gains people's interest now? :P. Anyway, it's good move to reduce the MMIO trap here.

Thanks,
Zhi.

-----Original Message-----
From: Chris Wilson [mailto:chris at chris-wilson.co.uk] 
Sent: Thursday, July 13, 2017 12:28 PM
To: Zhenyu Wang <zhenyuw at linux.intel.com>
Cc: intel-gfx at lists.freedesktop.org; Kuoppala, Mika <mika.kuoppala at intel.com>; Wang, Zhi A <zhi.a.wang at intel.com>
Subject: Re: [Intel-gfx] [PATCH v2 4/5] drm/i915/execlists: Read the context-status buffer from the HWSP

Quoting Zhenyu Wang (2017-07-13 10:14:34)
> On 2017.07.13 10:00:25 +0100, Chris Wilson wrote:
> > The engine provides a mirror of the CSB in the HWSP. If we use the 
> > cacheable reads from the HWSP, we can shave off a few mmio reads per 
> > context-switch interrupt (which are quite frequent!). Just removing 
> > a couple of mmio is not enough to actually reduce any latency, but a 
> > small reduction in overall cpu usage.
> 
> Unfortunately current gvt's execlist emulation depends on MMIO CSB 
> read for guest workload without guest HWSP update. So this can't work for guest.
> We need to fix that in gvt, also reduce MMIO trap is good benefit for vGPU too.
> But might have to fallback to mmio mode if vgpu active now, and once 
> gvt host got fixed, will notify through pvinfo to enable this.

* shakes fist at gvt, spoilsports.

Is the right test intel_vgpu_active()? i.e. something like

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index e3e9d850cf88..a259d584e1fd 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -504,6 +504,13 @@ static void intel_lrc_irq_handler(unsigned long data)
                        &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
                unsigned int head, tail;
 
+               /* However GVT emulation depends upon intercepting CSB mmio */
+               if (unlikely(intel_vgpu_active(dev_priv))) {
+                       buf = (u32 * __force)dev_priv->regs +
+                               i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine));
+                       engine->csb_head = -1;
+               }
+

would do the trick, though I guess it wants to test a version if GVT starts providing CSB emulation via the HWSP.
-Chris