Kernelspace C11 atomics for MIPS

[Warner Losh]

On Jun 3, 2013, at 8:45 PM, Adrian Chadd wrote:

> Speaking of this; any idea why the SYNC operators have 8 NOPs following them?

Yes, that's the exact issue that I've had with them, but have never had time to sort it out...

Warner

> I noticed that when going through disassemblies of various mips24k .o files.
> 
> 
> 
> Adrian
> 
> On 3 June 2013 10:53, Warner Losh <imp at 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 += 5" == "atomic_fetch_add(&a, 5)"), which is of course not
>>> 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 newer 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 for 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 fairly performance killing assembler instruction, how would you feel about allowing 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 performance in many places.
>> 
>> Don't commit the kern/bla.c standard change to conf/files, it looks to be bogus :)
>> 
>> Warner
>> 
>> _______________________________________________
>> freebsd-mips at freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-mips
>> To unsubscribe, send any mail to "freebsd-mips-unsubscribe at freebsd.org"