[Top][All Lists]

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

Re: [PATCH v2 3/5] s390x: topology: CPU topology objects and structures

From: Thomas Huth
Subject: Re: [PATCH v2 3/5] s390x: topology: CPU topology objects and structures
Date: Wed, 29 Sep 2021 10:12:02 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 07/09/2021 14.45, Pierre Morel wrote:

On 9/7/21 9:32 AM, Thomas Huth wrote:
On 22/07/2021 19.42, Pierre Morel wrote:
We use new objects to have a dynamic administration of the CPU topology.
The highier level object is the S390 book. In a first implementation

I didn't spot any migration related code in here ... is this already migration-safe?

Not sure at all.

The topology may change at any moment and we interpret PTF, the instruction which tell us if the topology changed.
Obviously the topology on the target may not be the same as on the source.

So what I propose is to disable topology change during the migration:
- on migration start, disable PTF interpretation and block the topology_change _report in the emulation.
- on migration end set back PTF interpretation and unblock the emulation

In the case, in discussion with David on KVM, that we do not emulate PTF for hosts without the stfl(11) we can even make it simpler in QEMU by always reporting "no change" for PTF 2 in the emulation.

Note that the Linux kernel, even if the topology can change at any moment use a polling every minute to check the topology changes, so I guess we can ignore the optimization during the migration.

What do you think?

I don't have much clue, this topology stuff is still mostly a black box to me - so there is no interrupt or something similar presented to the guest when the topology changes? The guest really has to poll for changes? ... that sounds like a weird design to me... if the guest polls too frequently, it wastes cycles due to the polling - but if it polls not often enough, it could run for a while with wrong topology information?


reply via email to

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