Re: [Qemu-devel] [FreeBSD] Host build i386 failing to build aarch64 targets

[Sean Bruno <sbruno@freebsd.org>]

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On 03/21/16 02:11, Alex Bennée wrote:
> 
> Peter Maydell <peter.maydell@linaro.org> writes:
> 
>> On 20 March 2016 at 19:20, Sean Bruno <sbruno@freebsd.org>
>> wrote:
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>>> 
>>> aarch64 targets are now failing to build on i386 hosts due to
>>> missing __atomic_load_8() calls since this commit:
>>> 
>>> https://github.com/qemu/qemu/commit/a0aa44b488b3601415d55041e4619aef5f3a4ba8#diff-c143d686899ae51d7b927d9c682e12fd
>>>
>>>
>>> 
I'm unsure if Linux is disabling aarch64 targets for i386 hosts or if
>>> this commit works "just fine" on Linux hosts right now, as it
>>> doesn't work with clang or gcc.
>> 
>> I think it just works on most Linux 32-bit architectures because 
>> the compiler support can inline a suitable atomic op (there is
>> one case where it doesn't, which I think is PPC32).
>> 
>> In any case, we mustn't use atomics on types larger than the host
>> pointer type, because it's not portable enough. Paolo or Alex,
>> can you have a look at this?
> 
> I'll get a BSD up and running and check. What is triggering the 
> __atomic_load_8 though?
> 
>> 

Specifically, its the atomic_read of vm_clock_warp_start in cpus.c:

 114 /***********************************************************/
 115 /* guest cycle counter */
 116
 117 /* Protected by TimersState seqlock */
 118
 119 static bool icount_sleep = true;
 120 static int64_t vm_clock_warp_start = -1;
 121 /* Conversion factor from emulated instructions to virtual clock
ticks.  */
 122 static int icount_time_shift;


....

 339 static void icount_warp_rt(void)
 340 {
 341     /* The icount_warp_timer is rescheduled soon after
vm_clock_warp_start
 342      * changes from -1 to another value, so the race here is okay.
 343      */
 344     if (atomic_read(&vm_clock_warp_start) == -1) {
 345         return;
 346     }
 347

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