[PATCH v5 07/15] drm: Add a prefetching memcpy_from_wc
Thomas Hellström
thomas.hellstrom at linux.intel.com
Fri May 28 15:10:50 UTC 2021
On 5/28/21 4:19 PM, Christian König wrote:
> Am 27.05.21 um 16:47 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 at ffwll.ch>
>> Cc: Christian König <christian.koenig at amd.com>
>> Suggested-by: Daniel Vetter <daniel at ffwll.ch>
>> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>> ---
>> Documentation/gpu/drm-mm.rst | 2 +-
>> drivers/gpu/drm/drm_cache.c | 138 +++++++++++++++++++++++++++++++++++
>> drivers/gpu/drm/drm_drv.c | 2 +
>> include/drm/drm_cache.h | 7 ++
>> 4 files changed, 148 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..08614f7fdd8d 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 +213,137 @@ bool drm_need_swiotlb(int dma_bits)
>> return max_iomem > ((u64)1 << dma_bits);
>> }
>> EXPORT_SYMBOL(drm_need_swiotlb);
>> +
>> +#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);
>> +}
>> +#endif
>> +
>> +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);
>> + }
>> + }
>> +}
>> +
>> +/**
>> + * 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()))
>> + return;
>
> I would either make it a BUG_ON() or at least use the fallback memcpy.
>
> Just returning without doing anything isn't really nice.
Hmm, Yes, Daniel suggested this on IRC. I would have gone for the
fallback which he didn't like, and I think crashing the kernel with a
BUG_ON in an interrupt which from experience might result in a
completely silent hang without a trace of what went wrong unless
possibly with a serial console is not really acceptable either....
Perhaps we can go for a WARN_ON + fallback, which still forces the
caller to come up with something else...
/Thomas
>
> Christian.
>
>> +
>> + if (IS_ENABLED(CONFIG_X86) &&
>> 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);
>> +
>> +#ifdef CONFIG_X86
>> +/**
>> + * 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_init_early(void)
>> +{
>> +}
>> +#endif
>> 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
>
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