Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub support

[Scott Wood <scottwood@freescale.com>]

On 04/02/2013 09:09:34 AM, Bhushan Bharat-R65777 wrote:
> 
> 
> > -----Original Message-----
> > From: Alexander Graf [mailto:agraf@suse.de]
> > Sent: Tuesday, April 02, 2013 1:57 PM
> > To: Bhushan Bharat-R65777
> > Cc: kvm-ppc@vger.kernel.org; kvm@vger.kernel.org; Wood Scott-B07421
> > Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub  
> support
> >
> >
> > On 29.03.2013, at 07:04, Bhushan Bharat-R65777 wrote:
> >
> > >
> > >
> > >> -----Original Message-----
> > >> From: Alexander Graf [mailto:agraf@suse.de]
> > >> Sent: Thursday, March 28, 2013 10:06 PM
> > >> To: Bhushan Bharat-R65777
> > >> Cc: kvm-ppc@vger.kernel.org; kvm@vger.kernel.org; Wood  
> Scott-B07421;
> > >> Bhushan
> > >> Bharat-R65777
> > >> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub
> > >> support
> > >>
> > >>
> > >> How does the normal debug register switching code work in Linux?
> > >> Can't we just reuse that? Or rely on it to restore working state  
> when
> > >> another process gets scheduled in?
> > >
> > > Good point, I can see debug registers loading in function  
> __switch_to()-
> > >switch_booke_debug_regs() in file arch/powerpc/kernel/process.c.
> > > So as long as assume that host will not use debug resources we  
> can rely on
> > this restore. But I am not sure that this is a fare assumption. As  
> Scott earlier
> > mentioned someone can use debug resource for kernel debugging also.
> >
> > Someone in the kernel can also use floating point registers. But  
> then it's his
> > responsibility to clean up the mess he leaves behind.
> 
> I am neither convinced by what you said and nor even have much reason  
> to oppose :)
> 
> Scott,
> 	I remember you mentioned that host can use debug resources, you  
> comment on this ?

I thought the conclusion we reached was that it was OK as long as KVM  
waits until it actually needs the debug resources to mess with the  
registers.

-Scott