Date: Sat, 3 Mar 2018 12:21:06 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Bruce Evans <brde@optusnet.com.au> Cc: Pedro Giffuni <pfg@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r330285 - head/sys/sys Message-ID: <20180303102106.GG3194@kib.kiev.ua> In-Reply-To: <20180303130511.N1283@besplex.bde.org> References: <201803021647.w22Gl2t7092316@repo.freebsd.org> <dba3b880-58d0-e6df-8f99-4d27919134c9@FreeBSD.org> <20180302181934.GF3194@kib.kiev.ua> <20180303130511.N1283@besplex.bde.org>
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On Sat, Mar 03, 2018 at 01:47:41PM +1100, Bruce Evans wrote: > On Fri, 2 Mar 2018, Konstantin Belousov wrote: > > > On Fri, Mar 02, 2018 at 12:43:34PM -0500, Pedro Giffuni wrote: > >> ... > >> I think use of _Nonnull attributes in the threading functions may also > >> be a waste (I introduced them mostly to be compatible with Android). > >> FWIW, Dragonfly sprinkled some restrict there recently: > >> > >> http://gitweb.dragonflybsd.org/dragonfly.git/commit/d33005aaee6af52c80428b59b52aee522c002492 > >> > >> Just in case someone is considering more cleanups. > > > > This is not a cleanup for me, but a needed change. Right now x86 > > copyouts are implemented in asm, so whatever damage is done to the > > prototypes, only effect is at the caller side. In my work, i386 copyouts > > are done in C, so it starts matter. > > That seems slow, especially for small sizes as are common for syscall args > (in 1 of my versions, copyin() of args is optimized to fuword() in a loop, > and fuword() is optimized to not use pcb_onfault, so it is not much more > than 1 memory access. However, in your i386 version this optimization > would be negative since the slow part is switching the map, so fuword() > should never be used to access multiple words). Yes. I already explained it in private, the current choice for i386 is either to be neglected very fast, or to get this change to still be a Tier 1 32 bit platform. The change is to make 4/4g split for UVA/KVA. In particular, the change ensures that it is possible to self-host i386 for forthcoming years, which is not practical for armv7 now and would be less so with clang grow. In other news, my system already boots single-user on SMP machine and I have torture tests like setting invalid %ss segment by sigreturn(2), work. There is (much) more to come, but I am happy how the patch progressed so far. > However, copyinstr() and > copystr() should never have been "optimized" by writing them in asm. On > x86, their asm is badly written so they are slower than simple C versions > except on 8088's and maybe 8086's and maybe on the original i386. (8088's > were limited mainly by instruction bandwidth and the original i386 wasn't > much better, so short CISC instructions like lodsb and stosb tended to be > faster than larger separate instructions despite their large setup overheads. Sure, copyinstr() is rewritten in C. The current version of copyout stuff is there: https://kib.kiev.ua/git/gitweb.cgi?p=deviant2.git;a=blob;f=sys/i386/i386/copyout.c;h=9747c06a84d7d2b5faac946f5de57f6a34d96c8c;hb=refs/heads/pcid > > > Also I looked at the dragonfly commit because I become curious what do you > > mean by threading functions. The first example was > > int pthread_attr_getguardsize(const pthread_attr_t * __restrict, > > - size_t *); > > + size_t * __restrict); > > POSIX agrees with the dragonfly change, but I do not understand it. > > Aliasing rules already disallow the first and second arguments to point > > to the same memory, because they have different types. > > (1) thread_attr_t is opaque, so the types might be the same. > (2) pthread_attr_t might be a pointer to a struct/union containing a size_t. > (3) perhaps other reasons. I'm not sure how 'restrict interacts with global > variables or even it it prevents the interaction in (2). A previous > discussion showed that const doesn't make types different enough to > prevent aliasing. Similarly for volatile. > > Similarly for other pointers to {opaque, struct/union, or even integer} types. > size_t can't be aliased to int, but it can be aliased to any unsigned type > in C and to any unsigned type not smaller than uint16_t in POSIX (POSIX > but not C requires u_char == uint8_t, so size_t can't be u_char in POSIX > but it can be u_char in C). I can only summarize it as 'there is no use to have restrict on the pthread_attr_getguardsize() arguments'.
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