On Wed, 1 Jul 2026 19:23:42 -0700, [email protected] wrote: > On Wed, Jul 01, 2026 at 05:05:52PM +0800, Li Zhe wrote: > > The new x86 memcpy_nt() helper in this series maps to > > memcpy_flushcache(), and the ZONE_DEVICE fast path uses that primitive > > for constant-sized struct page template copies. > > > > memcpy_flushcache() currently inlines only the 4, 8, and 16-byte > > cases. Larger constant-sized copies fall back to __memcpy_flushcache() > > even when the destination is naturally aligned. Extend the inline > > movnti coverage to 32, 48, 64, 80, and 96 bytes so the struct > > Why exactly those sizes and not bigger? > > Any numbers to back this up?
The relevant target sizes here are the x86_64 sizeof(struct page) values which patch 8 can hit: 64, 80, and 96 bytes. On x86_64, CONFIG_HAVE_ALIGNED_STRUCT_PAGE keeps struct page 16-byte aligned. That gives 64 bytes in the common case, 80 bytes when WANT_PAGE_VIRTUAL or CONFIG_KMSAN adds extra fields, and 96 bytes when both are present. I stopped there deliberately because this series does not have a current caller which needs larger constant sizes. The 32-byte and 48-byte cases were added only as intermediate fixed-size combinations used to build those larger struct page-sized copies while keeping them on the inline movnti path. > > page-sized copies used by that path can stay on the inline > > non-temporal store path instead of dropping into the out-of-line > > helper. > > > > Factor the store sequences into 8/16/32/64-byte helpers, keep the > > existing 4/8/16-byte cases handled directly in memcpy_flushcache(), > > issue the stores in ascending address order, and leave all other sizes > > on __memcpy_flushcache(). > > > > Signed-off-by: Li Zhe <[email protected]> > > --- > > arch/x86/include/asm/string_64.h | 80 +++++++++++++++++++++++++++++++- > > 1 file changed, 79 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/include/asm/string_64.h > > b/arch/x86/include/asm/string_64.h > > index 6f36abedc56a..95ef2d481418 100644 > > --- a/arch/x86/include/asm/string_64.h > > +++ b/arch/x86/include/asm/string_64.h > > @@ -4,6 +4,7 @@ > > > > #ifdef __KERNEL__ > > #include <linux/jump_label.h> > > +#include <linux/align.h> > > > > /* Written 2002 by Andi Kleen */ > > > > @@ -82,8 +83,81 @@ int strcmp(const char *cs, const char *ct); > > #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE > > #define __HAVE_ARCH_MEMCPY_FLUSHCACHE 1 > > void __memcpy_flushcache(void *dst, const void *src, size_t cnt); > > -static __always_inline void memcpy_flushcache(void *dst, const void *src, > > size_t cnt) > > + > > +static __always_inline void memcpy_flushcache_8(void *dst, const void *src) > > If this is calling MOVNTI, you might as well call it that way. Only leave the > externally exposed wrapper called "flushcache". Thanks for the review. In the next version I will rename the internal leaf helpers to movnti_*() names and keep memcpy_flushcache() as the externally exposed wrapper. > > +{ > > + asm volatile("movntiq %1, %0" > > + : "=m"(*(u64 *)dst) > > + : "r"(*(const u64 *)src) > > + : "memory"); > > +} > > + > > +static __always_inline void memcpy_flushcache_16(void *dst, > > + const void *src) > > +{ > > + memcpy_flushcache_8(dst, src); > > + memcpy_flushcache_8(dst + 8, src + 8); > > +} > > + > > +static __always_inline void memcpy_flushcache_32(void *dst, > > + const void *src) > > +{ > > + memcpy_flushcache_16(dst, src); > > + memcpy_flushcache_16(dst + 16, src + 16); > > +} > > + > > +static __always_inline void memcpy_flushcache_64(void *dst, > > + const void *src) > > { > > + memcpy_flushcache_32(dst, src); > > + memcpy_flushcache_32(dst + 32, src + 32); > > +} > > + > > +/* > > + * Keep the additional aligned fixed-size cases on the inline movnti path. > > + * Leave the existing 4/8/16-byte cases handled directly in > > + * memcpy_flushcache() so their code generation stays unchanged. > > + */ > > +static __always_inline int memcpy_flushcache_large(void *dst, > > + const void *src, > > + size_t cnt) > > +{ > > + char *dptr = dst; > > + const char *sptr = src; > > + > > + if (!IS_ALIGNED((unsigned long)dst, 8)) > > + return 0; > > + > > + switch (cnt) { > > + case 32: > > + memcpy_flushcache_32(dptr, sptr); > > + return 1; > > + case 48: > > + memcpy_flushcache_32(dptr, sptr); > > + memcpy_flushcache_16(dptr + 32, sptr + 32); > > + return 1; > > + case 64: > > + memcpy_flushcache_64(dptr, sptr); > > + return 1; > > + case 80: > > + memcpy_flushcache_64(dptr, sptr); > > + memcpy_flushcache_16(dptr + 64, sptr + 64); > > + return 1; > > + case 96: > > + memcpy_flushcache_64(dptr, sptr); > > + memcpy_flushcache_32(dptr + 64, sptr + 64); > > + return 1; > > + } > > + > > + return 0; > > +} > > + > > +static __always_inline void memcpy_flushcache(void *dst, const void *src, > > + size_t cnt) > > +{ > > + if (!cnt) > > + return; > > + > > if (__builtin_constant_p(cnt)) { > > switch (cnt) { > > case 4: > > @@ -97,7 +171,11 @@ static __always_inline void memcpy_flushcache(void > > *dst, const void *src, size_t > > asm ("movntiq %1, %0" : "=m"(*(u64 *)(dst + 8)) > > : "r"(*(u64 *)(src + 8))); > > return; > > } > > + > > + if (memcpy_flushcache_large(dst, src, cnt)) > > Why is this a separate function and not extending the switch-case? > > I betcha it has to do with the alignment check but nothing explains to me why. Yes. The separate helper is there exactly because the added 32/48/64/80/96-byte cases carry an extra 8-byte destination-alignment check, while I left the existing 4/8/16-byte inline cases unchanged. The intent was to keep the larger fixed-size cases on the inline movnti path only when they already match the alignment assumptions used by __memcpy_flushcache(); otherwise they fall back to that out-of-line helper, which already handles the misaligned head/tail fragments before entering its movnti loops. The current patch does not explain that clearly enough. In v6 I will add an explicit comment to memcpy_flushcache_large() and spell the same reasoning out in the changelog. Thanks, Zhe
