Re: [PATCH v5] drm/i915: Use SSE4.1 movntdqa to accelerate reads from WC memory

Matt Turner
2016-07-17T03:22:20Z

On Sat, Jul 16, 2016 at 8:44 AM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> This patch provides the infrastructure for performing a 16-byte aligned
> read from WC 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.
> (The CPU cache itself my be flushed for the region of the movntdqa and on
> later access the movntdqa reads from a separate internal buffer for the
> cacheline.) The write back to the memory is however cached.
>
> 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__.
> v4: Improve notes on CPU cache behaviour vs non-temporal stores.
> v5: Fix byte offsets for unrolled moves.
>
> 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..4ed4d3bb2f3e
> --- /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)

Many times, you've spelled this movntqda but the instruction mnemonic
is movntdqa (as seen below). I'm not sure if I'm missing something, or
if this typo has somehow slipped in so many times, including in the
commit message.

> +{
> + len >>= 4;
> + while (len >= 4) {
> + __asm__ __volatile__(
> + "movntdqa (%0), %%xmm0\n"
> + "movntdqa 16(%0), %%xmm1\n"
> + "movntdqa 32(%0), %%xmm2\n"
> + "movntdqa 48(%0), %%xmm3\n"
> + "movaps %%xmm0, (%1)\n"
> + "movaps %%xmm1, 16(%1)\n"
> + "movaps %%xmm2, 32(%1)\n"
> + "movaps %%xmm3, 48(%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 (unlikely(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
>
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx