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Re: [PATCH v14 08/11] qapi/s390/cpu topology: change-topology monitor co


From: Thomas Huth
Subject: Re: [PATCH v14 08/11] qapi/s390/cpu topology: change-topology monitor command
Date: Wed, 18 Jan 2023 11:53:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0

On 17/01/2023 14.31, Nina Schoetterl-Glausch wrote:
On Tue, 2023-01-17 at 08:30 +0100, Thomas Huth wrote:
On 16/01/2023 22.09, Nina Schoetterl-Glausch wrote:
On Thu, 2023-01-05 at 15:53 +0100, Pierre Morel wrote:
The modification of the CPU attributes are done through a monitor
commands.

It allows to move the core inside the topology tree to optimise
the cache usage in the case the host's hypervizor previously
moved the CPU.

The same command allows to modifiy the CPU attributes modifiers
like polarization entitlement and the dedicated attribute to notify
the guest if the host admin modified scheduling or dedication of a vCPU.

With this knowledge the guest has the possibility to optimize the
usage of the vCPUs.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
---
...
+    s390_topology.sockets[s390_socket_nb(id)]--;

I suppose this function cannot run concurrently, so the same CPU doesn't get 
removed twice.

QEMU has the so-called BQL - the Big Qemu Lock. Instructions handlers are
normally called with the lock taken, see qemu_mutex_lock_iothread() in
target/s390x/kvm/kvm.c.

That is good to know, but is that the relevant lock here?
We don't want to concurrent qmp commands. I looked at the code and it's pretty 
complicated.

Not sure, but I believe that QMP commands are executed from the main iothread, so I think this should be safe? ... CC:-ing some more people who might know the correct answer.

 Thomas




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