virgl: Flush context before waiting on fences

The logic behind this change is intuitive: if we are waiting for
something, we should probably flush all pending rendering so that it
starts executing in the meantime. This prevents the GPU from sitting
idle for long periods of time while we are also blocked in the app.

With the gun3d trace:
Before: 79 fps After: 215 fps

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13096>

src/gallium/drivers/virgl/virgl_context.c

@@ -936,8 +936,8 @@ static void virgl_submit_cmd(struct virgl_winsys *vws,
    }
 }
 
-static void virgl_flush_eq(struct virgl_context *ctx, void *closure,
-			   struct pipe_fence_handle **fence)
+void virgl_flush_eq(struct virgl_context *ctx, void *closure,
+		    struct pipe_fence_handle **fence)
 {
    struct virgl_screen *rs = virgl_screen(ctx->base.screen);
 

src/gallium/drivers/virgl/virgl_context.h

@@ -138,4 +138,6 @@ void
 virgl_rebind_resource(struct virgl_context *vctx,
                       struct pipe_resource *res);
 
+void virgl_flush_eq(struct virgl_context *ctx, void *closure,
+		    struct pipe_fence_handle **fence);
 #endif

src/gallium/drivers/virgl/virgl_screen.c

@@ -802,6 +802,10 @@ static bool virgl_fence_finish(struct pipe_screen *screen,
 {
    struct virgl_screen *vscreen = virgl_screen(screen);
    struct virgl_winsys *vws = vscreen->vws;
+   struct virgl_context *vctx = virgl_context(ctx);
+
+   if (vctx && timeout)
+      virgl_flush_eq(vctx, NULL, NULL);
 
    return vws->fence_wait(vws, fence, timeout);
 }