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Re: [qemu-s390x] [Qemu-devel] [PULL v3 47/55] linux headers: update agai


From: Alex Bennée
Subject: Re: [qemu-s390x] [Qemu-devel] [PULL v3 47/55] linux headers: update against Linux 5.2-rc1
Date: Wed, 22 May 2019 14:42:07 +0100
User-agent: mu4e 1.3.2; emacs 26.1

Aleksandar Markovic <address@hidden> writes:

> On May 22, 2019 2:24 PM, "Cornelia Huck" <address@hidden> wrote:
>>
>> On Wed, 22 May 2019 14:10:39 +0200
>> Laurent Vivier <address@hidden> wrote:
>>
>> > On 22/05/2019 14:07, Cornelia Huck wrote:
>> > > On Wed, 22 May 2019 13:47:25 +0200
>> > > Philippe Mathieu-Daudé <address@hidden> wrote:
>> > >
>> > >> On 5/21/19 5:28 PM, Cornelia Huck wrote:
>> > >>> commit a188339ca5a396acc588e5851ed7e19f66b0ebd9
>> > >>>
>> > >>> Signed-off-by: Cornelia Huck <address@hidden>
>> > >>> ---
>> > >> [...]
>> > >>>   #define __NR_mq_notify 184
>> > >>>   __SC_COMP(__NR_mq_notify, sys_mq_notify, compat_sys_mq_notify)
>> > >>>   #define __NR_mq_getsetattr 185
>> > >>> @@ -536,8 +567,10 @@ __SC_COMP(__NR_msgsnd, sys_msgsnd,
> compat_sys_msgsnd)
>> > >>>   __SYSCALL(__NR_semget, sys_semget)
>> > >>>   #define __NR_semctl 191
>> > >>>   __SC_COMP(__NR_semctl, sys_semctl, compat_sys_semctl)
>> > >>> +#if defined(__ARCH_WANT_TIME32_SYSCALLS) || __BITS_PER_LONG != 32
>> > >
>> > > Eww. It seems only aarch64 sets __ARCH_WANT_TIME32_SYSCALLS, and the
>> > > second condition probably catches others but not mipsel.
>> > >
>> > >>>   #define __NR_semtimedop 192
>> > >>> -__SC_COMP(__NR_semtimedop, sys_semtimedop, compat_sys_semtimedop)
>> > >>> +__SC_COMP(__NR_semtimedop, sys_semtimedop, sys_semtimedop_time32)
>> > >>> +#endif
>> > >>>   #define __NR_semop 193
>> > >>>   __SYSCALL(__NR_semop, sys_semop)
>> > >> [...]
>> > >>
>> > >> https://app.shippable.com/github/qemu/qemu/runs/1703/summary/console
>> > >>
>> > >> It seems this commit introduce a regression on mips32:
>> > >>
>> > >>    CC      mipsel-linux-user/linux-user/syscall.o
>> > >> ./linux-user/syscall.c: In function 'safe_semtimedop':
>> > >> ./linux-user/syscall.c:697:25: error: '__NR_semtimedop' undeclared
>> > >> (first use in this function)
>> > >>       return safe_syscall(__NR_##name, arg1, arg2, arg3, arg4); \
>> > >
>> > > So, we unconditionally deal with this syscall, i.e. we assume it is
>> > > always present? (I'm not sure of the logic in linux-user code.)
>> > >
>> >
>> > linux-user assumes it is present if __NR_msgsnd is present.
>>
>> Hm. The kernel change seems to break that assumption. Does anyone with
>> mips knowledge have an idea whether that was intentional (and the
>> linux-user code needs to be changed), or whether that's an issue on the
>> kernel side?
>>
>
> Hi, Cornelia.
>
> Thanks for your involving into this issue!
>
> It could be that (not-originating-from-MIPS) kernel commit:
>
> https://github.com/torvalds/linux/commit/1a787fc5ba18ac767e635c58d06a0b46876184e3
>
> made a mess with system call availability for MIPS (I will forward this to
> MIPS kernel maintainer Paul Burton). My impression is that this was not
> intentional, and is a temporary instability of kernel interface.

I think this stems from 2038 time bomb work. Eventually they want it to
be possible to build non-legacy kernels that don't expose time32 to the
outside world. As part of that new system calls are being introduced
where needed. The IPC syscall which orignally multiplexed a bunch of
these operations on some systems would eventually be potentially phased
out.

> However, I think that QEMU nevertheless should not make the assumption that
> if __NR_MSGSND, than semtimedop() is present. It could be true, but it is
> still just self-imposed belief in QEMU, kernel never guarantied such things.
>
> The alternative way of invoking via IPCV6 (else part of “ifdef
> __NR_MSGSND”) should work for MIPS in the present stage of headers and
> kernel.

Yeah I think #ifndef __NR_ipc would work for now. It shouldn't affect
architectures that never had the IPC call.

> As a side note, perhaps we shoul update kernel headers only off of stable
> kernel releases.

I guess that's a part of the tension for supporting new kernel APIs
quickly. At least 5.2-rc1 wasn't a random tree - you would expect the
external facing ABI to be stable after the merge window closed. It would
be nice to know what new features were being exposed though.

>
> Regards,
> Aleksandar


--
Alex Bennée



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