Re: [Qemu-devel] [PATCH v2 11/12] cpus: call cpu_update_icount on read

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 05/04/2017 15:25, Alex Bennée wrote:
>> This ensures each time the vCPU thread reads the icount we update the
>> master timer_state.qemu_icount field. This way as long as updates are
>> in BQL protected sections (which they should be) the main-loop can
>> never come to update the log and find time has gone backwards.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>  cpus.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/cpus.c b/cpus.c
>> index ff75af449a..63de033cc8 100644
>> --- a/cpus.c
>> +++ b/cpus.c
>> @@ -246,19 +246,17 @@ void cpu_update_icount(CPUState *cpu)
>>
>>  int64_t cpu_get_icount_raw(void)
>>  {
>> -    int64_t icount;
>>      CPUState *cpu = current_cpu;
>>
>> -    icount = atomic_read(&timers_state.qemu_icount);
>>      if (cpu && cpu->running) {
>>          if (!cpu->can_do_io) {
>>              fprintf(stderr, "Bad icount read\n");
>>              exit(1);
>>          }
>>          /* Take into account what has run */
>> -        icount += cpu_get_icount_executed(cpu);
>> +        cpu_update_icount(cpu);
>>      }
>> -    return icount;
>> +    return atomic_read(&timers_state.qemu_icount);
>>  }
>>
>>  /* Return the virtual CPU time, based on the instruction counter.  */
>>
>
> Maybe the update should be done in gen_io_start instead.  There
> shouldn't be any interference between vCPU and I/O threads except
> between gen_io_start and gen_io_end.

I'm not sure I follow. gen_io_start is a translation time thing. At
least here we ensure we update whenever the value is read.


--
Alex Bennée