Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP virtualization

[Pierre Morel]

On 23/08/2018 15:38, David Hildenbrand wrote:
> On 23.08.2018 15:22, Halil Pasic wrote:
> > On 08/23/2018 02:47 PM, Pierre Morel wrote:
> > > On 23/08/2018 13:12, David Hildenbrand wrote:
> > [..]
> > > > > > > I'm confused, which 128 bit?
> > > > > >
> > > > > >
> > > > > > Me too :) , I was assuming this block to be 128bit, but the qci block
> > > > > > has 128 bytes....
> > > > > >
> > > > > > And looking at arch/s390/include/asm/ap.h, there is a lot of information
> > > > > > contained that is definitely not of interest for CPU models...
> > > > > >
> > > > > > I wonder if there is somewhere defined which bits are reserved for
> > > > > > future features/facilities, compared to ap masks and such.
> > > > > >
> > > > > > This is really hard to understand/plan without access to documentation.
> > > > > >
> > > > > > You (Halil, Tony, Pier, ...) should have a look if what I described
> > > > > > related to PQAP(QCI) containing features that should get part of the CPU
> > > > > > model makes sense or not. For now I was thinking that there is some part
> > > > > > inside of QCI that is strictly reserved for facilities/features that we
> > > > > > can use.
> >
> > No there is no such part. The architecture documentation is quite confusing
> > with some aspects (e.g. persistence) of how exactly some of these features
> > work and are indicated. I'm having a hard time finding my opinion. I may
> > end up asking some questions later, but for now i have to think first.
> >
> > Just one hint. There is a programming note stating that if bit 2 of the
> > QCI block is one there is at least one AP card in the machine that actually
> > has APXA installed.
> >
> > I read the architecture so that the APXA has a 'cpu part' (if we are
> > doing APXA the cpu can't spec exception on certain bits not being zor9)
> > and a 'card(s) part'.
> >
> > Since the stuff seems quite difficult to sort out properly, I ask myself
> > are there real problems we must solve?
> >
> > This ultimately seems to be  about the migration, right? You say 'This helps
> > to catch nasty migration bugs (e.g. APXA suddenly disappearing).' at the very
> > beginning of the discussion. Yes, we don't have to have an vfio_ap device,
> > he guest can and will start looking for AP resources if
> > only the cpu model features installed. So the guest could observe
> > a disappearing APXA, but I don't think that would lead to problems (with
> > Linux at least).
> >
> > And there ain't much AP a guest can sanely do without if no AP resources
> > are there.
> >
> > I would really prefer not rushing a solution if we don't have to.
> >
> > > > What is apsc, qact, rc8a in the qci blocks? are the facility bits?
> > >
> > > Yes, facility bits concerning the AP instructions
> >
> > According to the current AR document rc8a ain't a facility but bits
> > 0-2 and 4-7 kind of are.
>
> Easy ( :) ) answer. Everything that is the CPU part should get into the
> CPU model. Everything that is AP specific not. If APXA is not a CPU
> facility, fine with me to leave it out.
>
> Ack to not rushing, but also ack to not leaving out important things.
> Ack that this stuff is hard to ficure out.

APXA is not a CPU part, it is a machine part (SIE) and a AP part (QCI,TAPQ),
it has no influence on CPU instructions but on the AP instructions.
Consequently, if I understood the definition correctly, it should not go 
in the CPU model.

Regards,
Pierre




--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany