Re: [PATCH v7 07/15] drm: Add a prefetching memcpy_from_wc

["Christian König" <christian.koenig@amd.com>]

Am 31.05.21 um 14:19 schrieb Thomas Hellström:
> Reading out of write-combining mapped memory is typically very slow
> since the CPU doesn't prefetch. However some archs have special
> instructions to do this.
>
> So add a best-effort memcpy_from_wc taking dma-buf-map pointer
> arguments that attempts to use a fast prefetching memcpy and
> otherwise falls back to ordinary memcopies, taking the iomem tagging
> into account.
>
> The code is largely copied from i915_memcpy_from_wc.
>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Christian König <christian.koenig@amd.com>
> Suggested-by: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Acked-by: Christian König <christian.koenig@amd.com>

> ---
> v7:
> - Perform a memcpy even if warning with in_interrupt(). Suggested by
>    Christian König.
> - Fix compilation failure on !X86 (Reported by kernel test robot
>    lkp@intel.com)
> ---
>   Documentation/gpu/drm-mm.rst |   2 +-
>   drivers/gpu/drm/drm_cache.c  | 147 +++++++++++++++++++++++++++++++++++
>   drivers/gpu/drm/drm_drv.c    |   2 +
>   include/drm/drm_cache.h      |   7 ++
>   4 files changed, 157 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index 21be6deadc12..c66058c5bce7 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -469,7 +469,7 @@ DRM MM Range Allocator Function References
>   .. kernel-doc:: drivers/gpu/drm/drm_mm.c
>      :export:
>   
> -DRM Cache Handling
> +DRM Cache Handling and Fast WC memcpy()
>   ==================
>   
>   .. kernel-doc:: drivers/gpu/drm/drm_cache.c
> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> index 79a50ef1250f..b887d7dec8b8 100644
> --- a/drivers/gpu/drm/drm_cache.c
> +++ b/drivers/gpu/drm/drm_cache.c
> @@ -28,6 +28,7 @@
>    * Authors: Thomas Hellström <thomas-at-tungstengraphics-dot-com>
>    */
>   
> +#include <linux/dma-buf-map.h>
>   #include <linux/export.h>
>   #include <linux/highmem.h>
>   #include <linux/mem_encrypt.h>
> @@ -35,6 +36,9 @@
>   
>   #include <drm/drm_cache.h>
>   
> +/* A small bounce buffer that fits on the stack. */
> +#define MEMCPY_BOUNCE_SIZE 128
> +
>   #if defined(CONFIG_X86)
>   #include <asm/smp.h>
>   
> @@ -209,3 +213,146 @@ bool drm_need_swiotlb(int dma_bits)
>   	return max_iomem > ((u64)1 << dma_bits);
>   }
>   EXPORT_SYMBOL(drm_need_swiotlb);
> +
> +static void memcpy_fallback(struct dma_buf_map *dst,
> +			    const struct dma_buf_map *src,
> +			    unsigned long len)
> +{
> +	if (!dst->is_iomem && !src->is_iomem) {
> +		memcpy(dst->vaddr, src->vaddr, len);
> +	} else if (!src->is_iomem) {
> +		dma_buf_map_memcpy_to(dst, src->vaddr, len);
> +	} else if (!dst->is_iomem) {
> +		memcpy_fromio(dst->vaddr, src->vaddr_iomem, len);
> +	} else {
> +		/*
> +		 * Bounce size is not performance tuned, but using a
> +		 * bounce buffer like this is significantly faster than
> +		 * resorting to ioreadxx() + iowritexx().
> +		 */
> +		char bounce[MEMCPY_BOUNCE_SIZE];
> +		void __iomem *_src = src->vaddr_iomem;
> +		void __iomem *_dst = dst->vaddr_iomem;
> +
> +		while (len >= MEMCPY_BOUNCE_SIZE) {
> +			memcpy_fromio(bounce, _src, MEMCPY_BOUNCE_SIZE);
> +			memcpy_toio(_dst, bounce, MEMCPY_BOUNCE_SIZE);
> +			_src += MEMCPY_BOUNCE_SIZE;
> +			_dst += MEMCPY_BOUNCE_SIZE;
> +			len -= MEMCPY_BOUNCE_SIZE;
> +		}
> +		if (len) {
> +			memcpy_fromio(bounce, _src, MEMCPY_BOUNCE_SIZE);
> +			memcpy_toio(_dst, bounce, MEMCPY_BOUNCE_SIZE);
> +		}
> +	}
> +}
> +
> +#ifdef CONFIG_X86
> +
> +static DEFINE_STATIC_KEY_FALSE(has_movntdqa);
> +
> +static void __memcpy_ntdqa(void *dst, const void *src, unsigned long len)
> +{
> +	kernel_fpu_begin();
> +
> +	while (len >= 4) {
> +		asm("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("movntdqa (%0), %%xmm0\n"
> +		    "movaps %%xmm0, (%1)\n"
> +		    :: "r" (src), "r" (dst) : "memory");
> +		src += 16;
> +		dst += 16;
> +	}
> +
> +	kernel_fpu_end();
> +}
> +
> +/*
> + * __drm_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.
> + */
> +static void __drm_memcpy_from_wc(void *dst, const void *src, unsigned long len)
> +{
> +	if (unlikely(((unsigned long)dst | (unsigned long)src | len) & 15))
> +		memcpy(dst, src, len);
> +	else if (likely(len))
> +		__memcpy_ntdqa(dst, src, len >> 4);
> +}
> +
> +/**
> + * drm_memcpy_from_wc - Perform the fastest available memcpy from a source
> + * that may be WC.
> + * @dst: The destination pointer
> + * @src: The source pointer
> + * @len: The size of the area o transfer in bytes
> + *
> + * Tries an arch optimized memcpy for prefetching reading out of a WC region,
> + * and if no such beast is available, falls back to a normal memcpy.
> + */
> +void drm_memcpy_from_wc(struct dma_buf_map *dst,
> +			const struct dma_buf_map *src,
> +			unsigned long len)
> +{
> +	if (WARN_ON(in_interrupt())) {
> +		memcpy_fallback(dst, src, len);
> +		return;
> +	}
> +
> +	if (static_branch_likely(&has_movntdqa)) {
> +		__drm_memcpy_from_wc(dst->is_iomem ?
> +				     (void __force *)dst->vaddr_iomem :
> +				     dst->vaddr,
> +				     src->is_iomem ?
> +				     (void const __force *)src->vaddr_iomem :
> +				     src->vaddr,
> +				     len);
> +		return;
> +	}
> +
> +	memcpy_fallback(dst, src, len);
> +}
> +EXPORT_SYMBOL(drm_memcpy_from_wc);
> +
> +/**
> + * drm_memcpy_init_early - One time initialization of the WC memcpy code
> + */
> +void drm_memcpy_init_early(void)
> +{
> +	/*
> +	 * Some hypervisors (e.g. KVM) don't support VEX-prefix instructions
> +	 * emulation. So don't enable movntdqa in hypervisor guest.
> +	 */
> +	if (static_cpu_has(X86_FEATURE_XMM4_1) &&
> +	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		static_branch_enable(&has_movntdqa);
> +}
> +#else
> +void drm_memcpy_from_wc(struct dma_buf_map *dst,
> +			const struct dma_buf_map *src,
> +			unsigned long len)
> +{
> +	WARN_ON(in_interrupt());
> +
> +	memcpy_fallback(dst, src, len);
> +}
> +
> +void drm_memcpy_init_early(void)
> +{
> +}
> +#endif /* CONFIG_X86 */
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 3d8d68a98b95..8804ec7d3215 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -35,6 +35,7 @@
>   #include <linux/slab.h>
>   #include <linux/srcu.h>
>   
> +#include <drm/drm_cache.h>
>   #include <drm/drm_client.h>
>   #include <drm/drm_color_mgmt.h>
>   #include <drm/drm_drv.h>
> @@ -1041,6 +1042,7 @@ static int __init drm_core_init(void)
>   
>   	drm_connector_ida_init();
>   	idr_init(&drm_minors_idr);
> +	drm_memcpy_init_early();
>   
>   	ret = drm_sysfs_init();
>   	if (ret < 0) {
> diff --git a/include/drm/drm_cache.h b/include/drm/drm_cache.h
> index e9ad4863d915..cc9de1632dd3 100644
> --- a/include/drm/drm_cache.h
> +++ b/include/drm/drm_cache.h
> @@ -35,6 +35,8 @@
>   
>   #include <linux/scatterlist.h>
>   
> +struct dma_buf_map;
> +
>   void drm_clflush_pages(struct page *pages[], unsigned long num_pages);
>   void drm_clflush_sg(struct sg_table *st);
>   void drm_clflush_virt_range(void *addr, unsigned long length);
> @@ -70,4 +72,9 @@ static inline bool drm_arch_can_wc_memory(void)
>   #endif
>   }
>   
> +void drm_memcpy_init_early(void);
> +
> +void drm_memcpy_from_wc(struct dma_buf_map *dst,
> +			const struct dma_buf_map *src,
> +			unsigned long len);
>   #endif