[PATCH v3] drm/i915: Use SSE4.1 movntqda to accelerate reads from WC memory

Chris Wilson
2016-07-16T12:53:19Z

This patch provides the infrastructure for performing a 16-byte aligned
read from WC (or even uncached) memory using non-temporal instructions
introduced with sse4.1. Using movntdqa we can bypass the CPU caches and
read directly from memory and ignoring the page attributes set on the
CPU PTE i.e. negating the impact of an otherwise UC access, copying
using movntqda from WC is almost as fast as reading from WB memory,
modulo the possibility of both hitting the CPU cache or leaving the data
in the CPU cache for the next consumer.

This will be used in later patches to accelerate accessing WC memory.

v2: Report whether the accelerated copy is successful/possible.
v3: Function alignment override was only necessary when using the
function target("sse4.1") - which is not necessary for emitting movntdqa
from __asm__.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Akash Goel <akash.goel@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
drivers/gpu/drm/i915/Makefile | 3 ++
drivers/gpu/drm/i915/i915_drv.c | 2 +
drivers/gpu/drm/i915/i915_drv.h | 3 ++
drivers/gpu/drm/i915/i915_memcpy.c | 75 ++++++++++++++++++++++++++++++++++++++
4 files changed, 83 insertions(+)
create mode 100644 drivers/gpu/drm/i915/i915_memcpy.c

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 75318ebb8d25..a53853daa998 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -3,12 +3,15 @@
# Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.

subdir-ccflags-$(CONFIG_DRM_I915_WERROR) := -Werror
+subdir-ccflags-y += \
+ $(call as-instr,movntdqa (%eax)$(comma)%xmm0,-DCONFIG_AS_MOVNTDQA)

# Please keep these build lists sorted!

# core driver code
i915-y := i915_drv.o \
i915_irq.o \
+ i915_memcpy.o \
i915_params.o \
i915_pci.o \
i915_suspend.o \
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index c5b7b8e0678a..16612542496a 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -848,6 +848,8 @@ static int i915_driver_init_early(struct drm_i915_private *dev_priv,
mutex_init(&dev_priv->wm.wm_mutex);
mutex_init(&dev_priv->pps_mutex);

+ i915_memcpy_init_early(dev_priv);
+
ret = i915_workqueues_init(dev_priv);
if (ret < 0)
return ret;
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 27d9b2c374b3..3c266e7866ba 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -4070,4 +4070,7 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
return false;
}

+void i915_memcpy_init_early(struct drm_i915_private *dev_priv);
+bool i915_memcpy_from_wc(void *dst, const void *src, unsigned long len);
+
#endif
diff --git a/drivers/gpu/drm/i915/i915_memcpy.c b/drivers/gpu/drm/i915/i915_memcpy.c
new file mode 100644
index 000000000000..8370cdef15f6
--- /dev/null
+++ b/drivers/gpu/drm/i915/i915_memcpy.c
@@ -0,0 +1,75 @@
+#include "i915_drv.h"
+
+DEFINE_STATIC_KEY_FALSE(has_movntqa);
+
+#ifdef CONFIG_AS_MOVNTDQA
+static void __movntqda(void *dst, const void *src, unsigned long len)
+{
+ len >>= 4;
+ while (len >= 4) {
+ __asm__ __volatile__(
+ "movntdqa (%0), %%xmm0\n"
+ "movntdqa 8(%0), %%xmm1\n"
+ "movntdqa 16(%0), %%xmm2\n"
+ "movntdqa 24(%0), %%xmm3\n"
+ "movaps %%xmm0, (%1)\n"
+ "movaps %%xmm1, 8(%1)\n"
+ "movaps %%xmm2, 16(%1)\n"
+ "movaps %%xmm3, 24(%1)\n"
+ : : "r" (src), "r" (dst) : "memory");
+ src += 64;
+ dst += 64;
+ len -= 4;
+ }
+ while (len--) {
+ __asm__ __volatile__(
+ "movntdqa (%0), %%xmm0\n"
+ "movaps %%xmm0, (%1)\n"
+ : : "r" (src), "r" (dst) : "memory");
+ src += 16;
+ dst += 16;
+ }
+}
+#endif
+
+/**
+ * i915_memcpy_from_wc: perform an accelerated *aligned* read from WC
+ * @dst: destination pointer
+ * @src: source pointer
+ * @len: how many bytes to copy
+ *
+ * i915_memcpy_from_wc copies @len bytes from @src to @dst using
+ * non-temporal instructions where available. Note that all arguments
+ * (@src, @dst) must be aligned to 16 bytes and @len must be a multiple
+ * of 16.
+ *
+ * To test whether accelerated reads from WC are supported, use
+ * i915_memcpy_from_wc(NULL, NULL, 0);
+ *
+ * Returns true if the copy was successful, false if the preconditions
+ * are not met.
+ */
+bool i915_memcpy_from_wc(void *dst, const void *src, unsigned long len)
+{
+ GEM_BUG_ON((unsigned long)dst & 15);
+ GEM_BUG_ON((unsigned long)src & 15);
+
+ if (len & 15)
+ return false;
+
+#ifdef CONFIG_AS_MOVNTDQA
+ if (static_branch_likely(&has_movntqa)) {
+ if (len)
+ __movntqda(dst, src, len);
+ return true;
+ }
+#endif
+
+ return false;
+}
+
+void i915_memcpy_init_early(struct drm_i915_private *dev_priv)
+{
+ if (static_cpu_has(X86_FEATURE_XMM4_1))
+ static_branch_enable(&has_movntqa);
+}
--
2.8.1