Re: [Qemu-devel] [PATCH 5/8] timers: prepare the code for future races in calling qemu_clock_warp

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/10/2013 19:08, Alex Bligh ha scritto:
> 
> On 8 Oct 2013, at 17:56, Paolo Bonzini wrote:
> 
>>> Arguably the patch could document why removing the check for deadline > INT32_MAX
>>> (the bug for bug compatibility) is safe, as I couldn't entirely convince myself it
>>> was, mostly because I couldn't see why it was doing it in the first place.
>>
>> I couldn't convince myself that it is _not_ safe :) and it made the code
>> more complicated.  As soon as a deadline appears, qemu_clock_warp() will
>> be called again and update the icount_warp_timer.
>>
>> Ok to move that to a separate patch?
> 
> To be honest I put it in out of an abundance of caution. I am very
> tempted to take it out and see what breaks. As far as I can see all
> the time arithmetic is not int64_t; perhaps this was not always the
> case. I was more checking you hadn't removed it by accident. Perhaps
> just add "special casing deadlines > INT32_MAX removed as all
> arithmetic now 64 bit".
> 
> There is another offender in tcg_cpu_exec.
> 
>         deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);
> 
>         /* Maintain prior (possibly buggy) behaviour where if no deadline
>          * was set (as there is no QEMU_CLOCK_VIRTUAL timer) or it is more than
>          * INT32_MAX nanoseconds ahead, we still use INT32_MAX
>          * nanoseconds.
>          */
>         if ((deadline < 0) || (deadline > INT32_MAX)) {
>             deadline = INT32_MAX;
>         }
> 
>         count = qemu_icount_round(deadline);
>         qemu_icount += count;
>         decr = (count > 0xffff) ? 0xffff : count;
>         count -= decr;
>         env->icount_decr.u16.low = decr;
>         env->icount_extra = count;
> 
> This implies that qemu_icount_round() cannot take a 64 bit int.
> 
> static int64_t qemu_icount_round(int64_t count)
> {
>     return (count + (1 << icount_time_shift) - 1) >> icount_time_shift;
> }
> 
> I would have thought it better if qemu_icount_round just
> dealt sensibly with negative parameters.
> 

I'll clean that up separately.

Thanks,

Paolo