qemu-s390x
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


From: Christian Borntraeger
Subject: Re: [PATCH v13 0/7] s390x: CPU Topology
Date: Tue, 13 Dec 2022 14:41:49 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0



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.



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.



reply via email to

[Prev in Thread] Current Thread [Next in Thread]