Re: [Qemu-devel] [PATCH 5/8] s390x/kvm: Implemented SIGP START

[Alexander Graf]

On 17.12.2013, at 15:26, Thomas Huth <address@hidden> wrote:

> 
> Hi Alex,
> 
> On Tue, 17 Dec 2013 14:56:33 +0100
> Alexander Graf <address@hidden> wrote:
> 
>> 
>> On 17.12.2013, at 14:22, Jens Freimann <address@hidden> wrote:
>> 
>>> From: Thomas Huth <address@hidden>
>>> 
>>> This patch adds the missing START order to the SIGP instruction handler.
>> 
>> Does the spec define what happens on START when the CPU is already running?
> 
> As far as I can see, the spec does not explicitely defines the behavior
> in that case, it says only: "The order is effective only when the
> addressed CPU is in the stopped state". But according to my experiments
> (I wrote a test program that I ran without additional hypervisor), the
> START order is simply ignored when the CPU is already running and CC0 is
> returned.
> 
> This is also what happens with the code below, since
> s390_add_running_cpu() only does something if the "halted" flag is set,
> and kicking a CPU that is already running does not hurt either, as far
> as I can see.

Yup :)

> 
>> Does START modify any register state?
> 
> No, the CPU simply continues running with the state where it has been
> stopped before.

Ok, great. Then it really does the same thing :)


Alex