Re: [PATCH liburing 0/3] Add x86 32-bit support for the nolibc build

[Jens Axboe <axboe@kernel.dk>]

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