Date: Sat, 3 Mar 2018 10:06:41 -0500 From: Pedro Giffuni <pfg@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, Bruce Evans <brde@optusnet.com.au> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r330285 - head/sys/sys Message-ID: <0a205e40-16d3-863a-301c-cc537023ecb9@FreeBSD.org> In-Reply-To: <20180303102106.GG3194@kib.kiev.ua> References: <201803021647.w22Gl2t7092316@repo.freebsd.org> <dba3b880-58d0-e6df-8f99-4d27919134c9@FreeBSD.org> <20180302181934.GF3194@kib.kiev.ua> <20180303130511.N1283@besplex.bde.org> <20180303102106.GG3194@kib.kiev.ua>
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On 03/03/2018 05:21, Konstantin Belousov wrote: > 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. Very nice. >> 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'. > Well, I'll admit I don't understand well the advantages and that's why I brought up a pointer to the changes instead of working on them. Usually, standards compliance is reason enough for such change though. Pedro.
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