On Mon, 13 May 2019 13:34:35 +0200 David Hildenbrand <da...@redhat.com> wrote:
> On 13.05.19 12:55, Christian Borntraeger wrote: > > > > > > On 13.05.19 11:57, David Hildenbrand wrote: > >> On 13.05.19 11:51, Christian Borntraeger wrote: > >>> > >>> > >>> On 13.05.19 11:40, David Hildenbrand wrote: > >>>> On 13.05.19 11:34, Christian Borntraeger wrote: > >>>>> > >>>>> > >>>>> On 13.05.19 10:03, David Hildenbrand wrote: > >>>>>>>> + if ((SCCB_SIZE - sizeof(ReadInfo)) / sizeof(CPUEntry) < > >>>>>>>> S390_MAX_CPUS) > >>>>>>>> + mc->max_cpus = S390_MAX_CPUS - 8; > >>>>>>> > >>>>>>> This is too complicated, just set it always to 240. > >>>>>>> > >>>>>>> However, I am still not sure how to best handle this scenario. One > >>>>>>> solution is > >>>>>>> > >>>>>>> 1. Set it statically to 240 for machine > 4.1 > >>>>>>> 2. Keep the old machines unmodifed > >>>>>>> 3. Don't indicate the CPU feature for machines <= 4.0 > >>>>>>> > >>>>>>> #3 is the problematic part, as it mixes host CPU features and > >>>>>>> machines. > >>>>>>> Bad. The host CPU model should always look the same on all machines. I > >>>>>>> don't like this. > >>>>>>> > >>>>>> > >>>>>> FWIW, #3 is only an issue when modeling it via the CPU model, like > >>>>>> Christian suggested. > >>>>>> > >>>>>> I suggest the following > >>>>>> > >>>>>> 1. Set the max #cpus for 4.1 to 240 (already done) > >>>>>> 2. Keep it for the other machines unmodified (as suggested by Thomas) > >>>>>> 3. Create the layout of the SCCB depending on the machine type (to be > >>>>>> done) > >>>>>> > >>>>>> If we want to model diag318 via a CPU feature (which makes sense for > >>>>>> migration): > >>>>>> > >>>>>> 4. Disable diag318 with a warning if used with a machine < 4.1 > >>>>>> > >>>>> > >>>>> I think there is a simpler solution. It is perfectly fine to fail the > >>>>> startup > >>>>> if we cannot fulfil the cpu model. So lets just allow 248 and allow > >>>>> this feature > >>>>> also for older machines. And if somebody chooses both at the same time, > >>>>> lets fails the startup. > >>>> > >>>> To which knob do you want to glue the layout of the SCLP response? Like > >>>> I described? Do you mean instead of warning and masking the feature off > >>>> as I suggested, simply failing? > >>> > >>> The sclp response will depend on the dia318 cpu model flag. If its on, > >>> the sclp > >>> response will have it, otherwise not. > >>> - host-passthrough: not migration safe anyway > >>> - host-model: if the target has diag318 good, otherwise we reject > >>> migration > >>>> > >>>> In that case, -machine ..-4.0 -cpu host will not work on new HW with new > >>>> KVM. Just noting. > >>> > >>> Only if you have 248 CPUs (which is unlikely). My point was to do that > >>> for all > >>> machine levels. > >>> > >> > >> The issue with this approach is that e.g. libvirt is not aware of this > >> restriction. It could query "max_cpus" and expand the host-cpu model, > >> but starting a guest with > 240 cpus would fail. Maybe this is acceptable. > >> > > > > As of today we do the cpu model check in the same way. libvirt actually > > tries > > to run QEMU and handles failures. > > > > For a failure, the user still has still to use >240 CPUs in its XML. The > > only downside > > is that libvirt will not reject this right away. > > > > During startup we would then print an error message like > > > > "The diag318 cpu feature is only supported for 240 and less CPUs." > > > > This is of similar quality as > > "Selected CPU GA level is too new. Maximum supported model in the > > configuration: \'%s\'", > > > > But that can be tested using the runability information if I am not wrong. You mean the cpu level information, right? > > > and others that we have today. > > > > So yes, I think this would be acceptable. > > I guess it is acceptable yes. I doubt anybody uses that many CPUs in > production either way. But you never know. I think that using that many cpus is a more uncommon setup, but I still think that having to wait for actual failure is worse than being able to find out beforehand. Any way to make this discoverable?
