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

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> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Tuesday, April 02, 2013 11:30 PM
> To: Bhushan Bharat-R65777
> Cc: Alexander Graf; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood Scott-
> B07421
> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub support
> 
> On 04/02/2013 09:09:34 AM, Bhushan Bharat-R65777 wrote:
> >
> >
> > > -----Original Message-----
> > > From: Alexander Graf [mailto:agraf@xxxxxxx]
> > > Sent: Tuesday, April 02, 2013 1:57 PM
> > > To: Bhushan Bharat-R65777
> > > Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; 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@xxxxxxx]
> > > >> Sent: Thursday, March 28, 2013 10:06 PM
> > > >> To: Bhushan Bharat-R65777
> > > >> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; 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.

Right,  Are we also agreeing on that KVM will not save/restore host debug context on vcpu_load/vcpu_put()? KVM will load its context in vcpu_load() if needed and on vcpu_put() it will clear DBCR0 and DBSR.

Thanks
-Bharat




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