Date: Mon, 3 Jun 2013 19:45:48 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: Ed Schouten <ed@80386.nl>, freebsd-mips@freebsd.org, freebsd-arch@freebsd.org Subject: Re: Kernelspace C11 atomics for MIPS Message-ID: <CAJ-Vmo=vNbT9majPCZ8ugzPsNzh46DTD4mMDX-cuxx9Og91ptw@mail.gmail.com> In-Reply-To: <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com> References: <CAJOYFBD502MYbkVR2hnVDTYWOvOUr15=OPyjotNvv%2BZ09vQ1OQ@mail.gmail.com> <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com>
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Speaking of this; any idea why the SYNC operators have 8 NOPs following the= m? I noticed that when going through disassemblies of various mips24k .o files= . Adrian On 3 June 2013 10:53, Warner Losh <imp@bsdimp.com> wrote: > > On Jun 3, 2013, at 8:04 AM, Ed Schouten wrote: > >> Hi, >> >> As of r251230, it should be possible to use C11 atomics in >> kernelspace, by including <sys/stdatomic.h>! Even when not using Clang >> (but GCC 4.2), it is possible to use quite a large portion of the API. >> A couple of limitations: >> >> - The memory order argument is simply ignored, making all the calls do >> a full memory barrier. >> - At least Clang allows you to do arithmetic on C11 atomics directly >> (e.g. "a +=3D 5" =3D=3D "atomic_fetch_add(&a, 5)"), which is of course n= ot >> possible to mimick. >> - The atomic functions only work on 1,2,4,8-byte types, which is >> probably a good thing. >> >> Amazingly, it turns out that it most of the architectures, with the >> exception of ARM and MIPS. To make MIPS work, we need to implement >> some of the __sync_* functions that are described here: >> >> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html >> >> Some time ago I already added some of these functions to our >> libcompiler-rt in userspace, to make atomics work there. >> Unfortunately, these functions were quite horribly implemented, as I >> tried to build them on top of <machine/atomic.h>, which is far from >> trivial/efficient. It is also restricted to 4 and 8-byte types. That's >> why I thought: why not spend some time learning MIPS assembly and >> write some decent implementations for these functions? >> >> The result: >> >> http://80386.nl/pub/mips-stdatomic.txt > > The number of necessary syncs varies by processor type. There's also newe= r synchronization instructions that make this as efficient as possible for = all mips32r2 and mips64r2-based machines. Older Caviums, at least and maybe= newer ones, also have their own variants. What you have will mostly work f= or the processors we have to support. mips_sync could therefore be better. = Doing it before AND after seems like overkill as well. Since sync is a fair= ly performance killing assembler instruction, how would you feel about allo= wing optimizations? > > This is my biggest single concern about the patch, but it also my current= biggest concern about the MIPS atomic operators in general. > >> For now, please focus on sys/mips/mips/stdatomic.c. It implements all >> the __sync_* functions called by <stdatomic.h> for 1, 2, 4 and 8 byte >> types. There is some testing code in there as well, which can be >> ignored. This code disassembles to the following: >> >> http://80386.nl/pub/mips-stdatomic-disasm.txt >> >> As I don't own a MIPS system myself, I was thinking about tinkering a >> bit with qemu to see whether these functions work properly. My >> questions are: >> >> - Does anyone have any comments on the C code and/or the machine code >> generated? Are there some nifty tricks I can apply to make the machine >> code more efficient that I am unaware o? >> - Is there anyone interested in testing this code a bit more >> thoroughly on physical hardware? >> - Would anyone mind if I committed this to HEAD? > > I have some cavium gear I can easily test on, and some other stuff I can = less-easily test on. > > It wouldn't be horrible to commit to head, but it would affect performanc= e in many places. > > Don't commit the kern/bla.c standard change to conf/files, it looks to be= bogus :) > > Warner > > _______________________________________________ > freebsd-mips@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-mips > To unsubscribe, send any mail to "freebsd-mips-unsubscribe@freebsd.org"
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