Re: io_uring failure on parisc (32-bit userspace and 64-bit kernel)

[Helge Deller <deller@gmx.de>]

On 2/12/23 20:42, Jens Axboe wrote:
> On 2/12/23 12:35?PM, Helge Deller wrote:
>> On 2/12/23 15:03, Helge Deller wrote:
>>> On 2/12/23 14:35, Jens Axboe wrote:
>>>> On 2/12/23 6:28?AM, Helge Deller wrote:
>>>>> On 2/12/23 14:16, Jens Axboe wrote:
>>>>>> On 2/12/23 2:47?AM, Helge Deller wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> We see io-uring failures on the parisc architecture with this testcase:
>>>>>>> https://github.com/axboe/liburing/blob/master/examples/io_uring-test.c
>>>>>>>
>>>>>>> parisc is always big-endian 32-bit userspace, with either 32- or 64-bit kernel.
>>>>>>>
>>>>>>> On a 64-bit kernel (6.1.11):
>>>>>>> deller@parisc:~$ ./io_uring-test test.file
>>>>>>> ret=0, wanted 4096
>>>>>>> Submitted=4, completed=1, bytes=0
>>>>>>> -> failure
>>>>>>>
>>>>>>> strace shows:
>>>>>>> io_uring_setup(4, {flags=0, sq_thread_cpu=0, sq_thread_idle=0, sq_entries=4, cq_entries=8, features=IORING_FEAT_SINGLE_MMAP|IORING_FEAT_NODROP|IORING_FEAT_SUBMIT_STABLE|IORING_FEAT_RW_CUR_POS|IORING_FEAT_CUR_PERSONALITY|IORING_FEAT_FAST_POLL|IORING_FEAT_POLL_32BITS|0x1f80, sq_off={head=0, tail=16, ring_mask=64, ring_entries=72, flags=84, dropped=80, array=224}, cq_off={head=32, tail=48, ring_mask=68, ring_entries=76, overflow=92, cqes=96, flags=0x58 /* IORING_CQ_??? */}}) = 3
>>>>>>> mmap2(NULL, 240, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0) = 0xf7522000
>>>>>>> mmap2(NULL, 256, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0x10000000) = 0xf6922000
>>>>>>> openat(AT_FDCWD, "libell0-dbgsym_0.56-2_hppa.deb", O_RDONLY|O_DIRECT) = 4
>>>>>>> statx(4, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=689308, ...}) = 0
>>>>>>> getrandom("\x5c\xcf\x38\x2d", 4, GRND_NONBLOCK) = 4
>>>>>>> brk(NULL)                               = 0x4ae000
>>>>>>> brk(0x4cf000)                           = 0x4cf000
>>>>>>> io_uring_enter(3, 4, 0, 0, NULL, 8)     = 0
>>>>>>>
>>>>>>>
>>>>>>> Running the same testcase on a 32-bit kernel (6.1.11) works:
>>>>>>> root@debian:~# ./io_uring-test test.file
>>>>>>> Submitted=4, completed=4, bytes=16384
>>>>>>> -> ok.
>>>>>>>
>>>>>>> strace:
>>>>>>> io_uring_setup(4, {flags=0, sq_thread_cpu=0, sq_thread_idle=0, sq_entries=4, cq_entries=8, features=IORING_FEAT_SINGLE_MMAP|IORING_FEAT_NODROP|IORING_FEAT_SUBMIT_STABLE|IORING_FEAT_RW_CUR_POS|IORING_FEAT_CUR_PERSONALITY|IORING_FEAT_FAST_POLL|IORING_FEAT_POLL_32BITS|0x1f80, sq_off={head=0, tail=16, ring_mask=64, ring_entries=72, flags=84, dropped=80, array=224}, cq_off={head=32, tail=48, ring_mask=68, ring_entries=76, overflow=92, cqes=96, flags=0x58 /* IORING_CQ_??? */}}) = 3
>>>>>>> mmap2(NULL, 240, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0) = 0xf6d4c000
>>>>>>> mmap2(NULL, 256, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0x10000000) = 0xf694c000
>>>>>>> openat(AT_FDCWD, "trace.dat", O_RDONLY|O_DIRECT) = 4
>>>>>>> statx(4, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=1855488, ...}) = 0
>>>>>>> getrandom("\xb2\x3f\x0c\x65", 4, GRND_NONBLOCK) = 4
>>>>>>> brk(NULL)                               = 0x15000
>>>>>>> brk(0x36000)                            = 0x36000
>>>>>>> io_uring_enter(3, 4, 0, 0, NULL, 8)     = 4
>>>>>>>
>>>>>>> I'm happy to test any patch if someone has an idea....
>>>>>>
>>>>>> No idea what this could be, to be honest. I tried your qemu vm image,
>>>>>> and it does boot, but it's missing keys to be able to update apt and
>>>>>> install packages... After fiddling with this for 30 min I gave up, any
>>>>>> chance you can update the sid image? Given how slow this thing is
>>>>>> running, it'd take me all day to do a fresh install and I have to admit
>>>>>> I'm not THAT motivated about parisc to do that :)
>>>>>
>>>>> Yes, I will update that image, but qemu currently only supports a
>>>>> 32-bit PA-RISC CPU which can only run the 32-bit kernel. So even if I
>>>>> update it, you won't be able to reproduce it, as it only happens with
>>>>> the 64-bit kernel. I'm sure it's some kind of missing 32-to-64bit
>>>>> translation in the kernel, which triggers only big-endian machines.
>>>>
>>>> I built my own kernel for it, so that should be fine, correct?
>>>
>>> No, as qemu won't boot the 64-bit kernel.
>>>
>>>> We'll see soon enough, managed to disable enough checks on the
>>>> debian-10 image to actually make it install packages.
>>>>
>>>>> Does powerpc with a 64-bit ppc64 kernel work?
>>>>> I'd assume it will show the same issue.
>>>>
>>>> No idea... Only stuff I use and test on is x86-64/32 and arm64.
>>>
>>> Would be interesting if someone could test...
>>>
>>>>> I will try to add some printks and compare the output of 32- and
>>>>> 64-bit kernels. If you have some suggestion where to add such (which?)
>>>>> debug code, it would help me a lot.
>>>>
>>>> I'd just try:
>>>>
>>>> echo 1 > /sys/kernel/debug/tracing/events/io_uring
>>>
>>> I'll try, but will take some time...
>>>
>>
>> At entry of io_submit_sqes(), io_sqring_entries() returns 0, because
>> ctx->rings->sq.tail is 0 (wrongly on broken 64-bit, but ok value 4 on 32-bit), and
>> ctx->cached_sq_head is 0 in both cases.
>
> cached_sq_head will get updated as sqes are consumed, but since sq.tail
> is zero, there's nothing to submit as far as io_uring is concerned.
>
> Can you dump addresses/offsets of the sq and cq heads/tails in userspace
> and in the kernel? They are u32, so same size of 32 and 64-bit.

For both kernels (32- and 64-bit) I get:
p->sq_off.head = 0  p->sq_off.tail = 16
p->cq_off.head = 32  p->cq_off.tail = 48

Helge