Comment 10 for bug 1344320

On 25 July 2014 10:54, Riku Voipio <email address hidden> wrote:
> On Tue, Jul 22, 2014 at 10:22:15PM -0000, Peter Maydell wrote:
>> On 22 July 2014 20:46, Richard Henderson <email address hidden> wrote:
>> > On 07/21/2014 10:37 AM, Peter Maydell wrote:
>> >>> It's trying to measure clock cycles required to perform the startup
>> >>> relocations.
>> >>
>> >> That's a neat trick, given that the generic timers are not cycle
>> >> counters! They're a fixed frequency counter which is generally
>> >> unrelated and rather slower than the CPU frequency (and
>> >> which doesn't scale up and down with CPU frequency either).
>> >
>> > Even better.
>>
>> In any case the kernel guys say you can't guarantee they
>> exist unless you get them to define an ELF hwcap for
>> "timers exist and have a sane value in the 'what frequency
>> are they' register". So this is a glibc bug and I'm
>> not fixing QEMU...
>
> It's used by openssl for rdtsc emulation as well:
>
> http://git.openssl.org/gitweb/?p=openssl.git;a=blob;f=crypto/arm64cpuid.S#l17
>

It is well known to me and to the upstream author that the timer count
is meaningless without also reading the frequency.
However, the piece of code you refer to is only used for internal
development by the upstream author, so I don't think we should care
about it.
(The snippet in question is executed with a SIGILL trap set as a kind
of poor man's elf_hwcap)

--
Ard.

>> -- PMM
>>
>> Title:
>> qemu-aarch64 cannot execute glibc
>>
>> Status in QEMU:
>> New
>>
>> Bug description:
>> $ aarch64-linux-user/qemu-aarch64 -version
>> qemu-aarch64 version 2.0.92, Copyright (c) 2003-2008 Fabrice Bellard
>> $ aarch64-linux-user/qemu-aarch64 -d in_asm /daten/build/build-root/home/abuild/rpmbuild/BUILD/glibc-2.19.90/cc-base/elf/ld-linux-aarch64.so.1
>> host mmap_min_addr=0x10000
>> guest_base 0x0
>> start end size prot
>> 0000004000000000-000000400001e000 000000000001e000 r-x
>> 000000400001e000-000000400002e000 0000000000010000 ---
>> 000000400002e000-0000004000031000 0000000000003000 rw-
>> 0000004000031000-0000004000032000 0000000000001000 ---
>> 0000004000032000-0000004000832000 0000000000800000 rw-
>> start_brk 0x0000000000000000
>> end_code 0x000000400001dbe0
>> start_code 0x0000004000000000
>> start_data 0x000000400002eba8
>> end_data 0x0000004000030008
>> start_stack 0x0000004000830a10
>> brk 0x0000004000030170
>> entry 0x00000040000012c0
>> ----------------
>> IN:
>> 0x00000040000012c0: 910003e0 mov x0, sp
>> 0x00000040000012c4: 94000d4f bl #+0x353c (addr 0x7fffb5bdad68)
>>
>> ----------------
>> IN: _dl_start
>> 0x0000004000004800: d11243ff sub sp, sp, #0x490 (1168)
>> 0x0000004000004804: a9ba7bfd stp x29, x30, [sp, #-96]!
>> 0x0000004000004808: 910003fd mov x29, sp
>> 0x000000400000480c: a9046bf9 stp x25, x26, [sp, #64]
>> 0x0000004000004810: a90153f3 stp x19, x20, [sp, #16]
>> 0x0000004000004814: a9025bf5 stp x21, x22, [sp, #32]
>> 0x0000004000004818: a90363f7 stp x23, x24, [sp, #48]
>> 0x000000400000481c: a90573fb stp x27, x28, [sp, #80]
>> 0x0000004000004820: aa0003fa mov x26, x0
>> 0x0000004000004824: d5033fdf isb
>> 0x0000004000004828: d53be040 mrs x0, (unknown)
>>
>> qemu: uncaught target signal 4 (Illegal instruction) - core dumped
>> Illegal instruction
>> $ objdump -d /daten/build/build-root/home/abuild/rpmbuild/BUILD/glibc-2.19.90/cc-base/elf/ld-linux-aarch64.so.1 | grep ' 4828:'
>> 4828: d53be040 mrs x0, cntvct_el0
>>
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