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:

> ----- Original Message -----
>> From: "Alex Bennée" <alex.bennee@linaro.org>
>> To: "Paolo Bonzini" <pbonzini@redhat.com>
>> Cc: dovgaluk@ispras.ru, rth@twiddle.net, "peter maydell" <peter.maydell@linaro.org>, qemu-devel@nongnu.org,
>> mttcg@greensocs.com, "fred konrad" <fred.konrad@greensocs.com>, "a rigo" <a.rigo@virtualopensystems.com>,
>> cota@braap.org, "bobby prani" <bobby.prani@gmail.com>, nikunj@linux.vnet.ibm.com, "Peter Crosthwaite"
>> <crosthwaite.peter@gmail.com>
>> Sent: Friday, April 7, 2017 7:35:29 PM
>> Subject: Re: [PATCH v2 11/12] cpus: call cpu_update_icount on read
>>
>>
>> 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.
>
> Sorry, I meant we should generate TCG opcodes for the translation in
> gen_io_start.

Ahh OK. I think this is 2.10 stuff though right?

>
> Paolo


--
Alex Bennée