Re: [RFC PATCH v3 8/8] target/s390x: Use start-powered-off CPUState property

[Thiago Jung Bauermann]

Cornelia Huck <cohuck@redhat.com> writes:

> On Thu, 30 Jul 2020 11:45:41 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
>
>> On Tue, 28 Jul 2020 21:51:33 -0300
>> Thiago Jung Bauermann <bauerman@linux.ibm.com> wrote:
>> 
>> > Hi,
>> > 
>> > Cornelia Huck <cohuck@redhat.com> writes:
>> >   
>> > > On Wed, 22 Jul 2020 23:56:57 -0300
>> > > Thiago Jung Bauermann <bauerman@linux.ibm.com> wrote:
>> > >    
>> > >> Instead of setting CPUState::halted to 1 in s390_cpu_initfn(), use the
>> > >> start-powered-off property which makes cpu_common_reset() initialize it
>> > >> to 1 in common code.
>> > >> 
>> > >> Note that this changes behavior by setting cs->halted to 1 on reset, 
>> > >> which
>> > >> didn't happen before.    
>> > >
>> > > I think that should be fine, as we change the cpu state to STOPPED in
>> > > the reset function, which sets halted to 1.    
>> > 
>> > Nice, thanks for checking.
>> >   
>> > >> 
>> > >> Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
>> > >> ---
>> > >>  target/s390x/cpu.c | 2 +-
>> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> > >> 
>> > >> NB: I was only able to test that this patch builds. I wasn't able to
>> > >> run it.    
>> > >
>> > > No noticeable difference under kvm, but running under tcg seems a bit
>> > > more sluggish than usual, and I saw some pausing on reboot (after the
>> > > bios handover to the kernel). Not sure if it were just flukes on my
>> > > laptop, would appreciate if someone else could give it a go.    
>> 
>> Experimented a bit with it again. There's a pause when switching from
>> the bios to the kernel (after the load reset normal has been done, I
>> guess), which is always there, but seems to get more noticeable with
>> this patch (varying wildly, but seems longer on average.) Hard to pin
>> down, and I don't really see a reason why that should happen, as we
>> should end up with halted == 1 in any case. Might still be a fluke,
>> even though I see it both on my laptop and on an LPAR (when running
>> under tcg; not seen under kvm, which is much faster anyway.)
>
> Tried again, the pause now seems comparable to the pause prior to this
> series. Might depend on the phase of the moon.
>
> I ran kvm unit tests on it, and it looks good. So, I'm reasonable
> confident that this is fine, really just seems to be a fluke.
>
> Acked-by: Cornelia Huck <cohuck@redhat.com>

Ah, that's a relief. Thank you very much for looking into this!

-- 
Thiago Jung Bauermann
IBM Linux Technology Center