Re: [PATCH v13 0/7] s390x: CPU Topology

[Janis Schoetterl-Glausch]

On Tue, 2022-12-13 at 14:41 +0100, Christian Borntraeger wrote:
> 
> Am 12.12.22 um 11:17 schrieb Thomas Huth:
> > On 12/12/2022 11.10, Pierre Morel wrote:
> > > 
> > > 
> > > On 12/12/22 10:07, Thomas Huth wrote:
> > > > On 12/12/2022 09.51, Pierre Morel wrote:
> > > > > 
> > > > > 
> > > > > On 12/9/22 14:32, Thomas Huth wrote:
> > > > > > On 08/12/2022 10.44, Pierre Morel wrote:
> > > > > > > Hi,
> > > > > > > 
> > > > > > > Implementation discussions
> > > > > > > ==========================
> > > > > > > 
> > > > > > > CPU models
> > > > > > > ----------
> > > > > > > 
> > > > > > > Since the S390_FEAT_CONFIGURATION_TOPOLOGY is already in the CPU 
> > > > > > > model
> > > > > > > for old QEMU we could not activate it as usual from KVM but needed
> > > > > > > a KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
> > > > > > > Checking and enabling this capability enables
> > > > > > > S390_FEAT_CONFIGURATION_TOPOLOGY.
> > > > > > > 
> > > > > > > Migration
> > > > > > > ---------
> > > > > > > 
> > > > > > > Once the S390_FEAT_CONFIGURATION_TOPOLOGY is enabled in the source
> > > > > > > host the STFL(11) is provided to the guest.
> > > > > > > Since the feature is already in the CPU model of older QEMU,
> > > > > > > a migration from a new QEMU enabling the topology to an old QEMU
> > > > > > > will keep STFL(11) enabled making the guest get an exception for
> > > > > > > illegal operation as soon as it uses the PTF instruction.
> > > > > > 
> > > > > > I now thought that it is not possible to enable "ctop" on older 
> > > > > > QEMUs since the don't enable the KVM capability? ... or is it still 
> > > > > > somehow possible? What did I miss?
> > > > > > 
> > > > > >   Thomas
> > > > > 
> > > > > Enabling ctop with ctop=on on old QEMU is not possible, this is right.
> > > > > But, if STFL(11) is enable in the source KVM by a new QEMU, I can see 
> > > > > that even with -ctop=off the STFL(11) is migrated to the destination.
> 
> This does not make sense. the cpu model and stfle values are not migrated. 
> This is re-created during startup depending on the command line parameters of 
> -cpu.
> Thats why source and host have the same command lines for -cpu. And STFLE.11 
> must not be set on the SOURCE of ctop is off.
> 
What about linux? I didn't look to thoroughly at it, but it caches the stfle 
bits, doesn't it?
So if the migration succeeds, even tho it should not, it will look to the guest 
like the facility is enabled.
> 
> > > > 
> > > > Is this with the "host" CPU model or another one? And did you 
> > > > explicitly specify "ctop=off" at the command line, or are you just 
> > > > using the default setting by not specifying it?
> > > 
> > > With explicit cpumodel and using ctop=off like in
> > > 
> > > sudo /usr/local/bin/qemu-system-s390x_master \
> > >       -m 512M \
> > >       -enable-kvm -smp 4,sockets=4,cores=2,maxcpus=8 \
> > >       -cpu z14,ctop=off \
> > >       -machine s390-ccw-virtio-7.2,accel=kvm \
> > >       ...
> > 
> > Ok ... that sounds like a bug somewhere in your patches or in the kernel 
> > code ... the guest should never see STFL bit 11 if ctop=off, should it?
> 
> Correct. If ctop=off then QEMU should disable STFLE.11 for the CPU model.