On 4/18/22 8:14 AM, Ammar Faizi wrote: > On 4/18/22 9:01 AM, Jens Axboe wrote: >> On 4/14/22 4:41 PM, Ammar Faizi wrote: >>> Hi, >>> >>> This series adds nolibc support for x86 32-bit. There are 3 patches in >>> this series: >>> >>> 1) Use `__NR_mmap2` instead of `__NR_mmap` for x86 32-bit. >>> 2) Provide `get_page_size()` function for x86 32-bit. >>> 3) Add x86 32-bit native syscall support. >>> >>> The most noticeable changes is the patch 3. Unlike x86-64, only >>> use native syscall from the __do_syscall macros when CONFIG_NOLIBC is >>> enabled for 32-bit build. The reason is because the libc syscall >>> wrapper can do better in 32-bit. The libc syscall wrapper can dispatch >>> the best syscall instruction that the environment is supported, there >>> are at least two variants syscall instruction for x86 32-bit, they are: >>> `int $0x80` and `sysenter`. The `int $0x80` instruction is always >>> available, but `sysenter` is not, it relies on VDSO. liburing always >>> uses `int $0x80` for syscall if it's compiled with CONFIG_NOLIBC, >>> otherwise it uses whatever the libc provides. >>> >>> Extra notes for __do_syscall6() macro: >>> On i386, the 6th argument of syscall goes in %ebp. However, both Clang >>> and GCC cannot use %ebp in the clobber list and in the "r" constraint >>> without using -fomit-frame-pointer. To make it always available for >>> any kind of compilation, the below workaround is implemented: >>> >>> 1) Push the 6-th argument. >>> 2) Push %ebp. >>> 3) Load the 6-th argument from 4(%esp) to %ebp. >>> 4) Do the syscall (int $0x80). >>> 5) Pop %ebp (restore the old value of %ebp). >>> 6) Add %esp by 4 (undo the stack pointer). >>> >>> WARNING: >>> Don't use register variables for __do_syscall6(), there is a known >>> GCC bug that results in an endless loop. >>> >>> BugLink: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105032 >>> >>> >>> ===== How is this tested? ===== >>> >>> This has been tested on x86-64 Linux (compiled with 32-bit bin support) >>> with the following commands: >>> >>> sudo apt-get install gcc-i686-linux-gnu g++-i686-linux-gnu -y; >>> ./configure --cc=i686-linux-gnu-gcc --cxx=i686-linux-gnu-g++ --nolibc; >>> sudo make -j8 runtests; >> >> Looks reasonable to me, even with the warts. I keep threatening to do a >> 2.2 release, and I do want to do that soon, so question is if we defer >> this patchset until after that has happened? >> >> I'm looking for a gauge of confidence on the series. > > I personally love not to defer this patchset. I understand that if we > were adding something like this to the Linux kernel, it's pretty sure > that it is not acceptable time. But liburing. > > Several things that you may want to consider: > > 1) Previously, `--nolibc` build on x86 32-bit will throw a compile > error, "Arch doesn't support building liburing without libc". > After this patchset, it compiles just fine. > > 2) This series doesn't have any effect for x86 32-bit that uses libc, > and that is what we do by default. > > 3) I believe x86 32-bit users are not that many. So having this one > earlier gives sometime to get it mature without much chaos (if > we ever found a bug). > > Not to say it's buggy. But younger code tend to be buggier. If we > ever hit that bug due to this patchset, some of them may fallback > to the libc build while waiting for the next stable liburing. > > But anyway, I don't think it's that urgent seeing that we don't have > visible users that are actively using nolibc x86 32-bit. So if you > prefer to defer this, please defer it. What do you think? All good points, let's just get it done. -- Jens Axboe
