[Qemu-devel] Re: [PATCH 0/9] S390x KVM support

[Alexander Graf]

On 22.10.2009, at 11:53, Avi Kivity wrote:

> On 10/22/2009 11:11 AM, Alexander Graf wrote:
> > > Doesn't this break backward compatibility by changing the structure?
> > >
> > > Best to put it after the union (and as a copy, so userspace that  
> > > expects the previous location still works).  If you're reading it  
> > > from the kernel, also need a way to tell the kernel which copy to  
> > > read from.
> > >
> > > Also advertise with a KVM_CAP.
> >
> >
> > I don't think we need to go through the hassle here. There is  
> > effectively no user of that code for now and the ABI is considered  
> > unstable.
>
> At the very least we need a KVM_CAP so qemu knows to fail on older  
> kernels.

Hm. Oh well :-). It can't hurt to have yet another CAP, right?

> > > I'd also appreciate an explanation of what this is all about.
> >
> > Explanation in the code or explanation in an email reply?
>
>
> email.  I assume s390 hackers would understand why the psw needs to  
> be exposed to qemu on every exit.  This is mostly for my personal  
> interest.

PSW = (eflags << 32) | pc;

:-)

Before that patch it was only synced with the "vmcb" on special  
userspace handled intercepts, now it's synced on every exit to  
userspace.

Alex