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Re: [Qemu-devel] [PATCH 0/3] add query-cpu-fast and related s390 changes

From: Viktor Mihajlovski
Subject: Re: [Qemu-devel] [PATCH 0/3] add query-cpu-fast and related s390 changes
Date: Mon, 12 Feb 2018 17:26:12 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 12.02.2018 16:38, Cornelia Huck wrote:
> On Mon, 12 Feb 2018 13:14:29 +0100
> Viktor Mihajlovski <address@hidden> wrote:
>> This series consolidates patches around a performance issue
>> caused by the usage of QMP query-cpus.
> Thank you for consolidating this; it was a bit hard to follow the
> different discussions.
>> A performance issue was found in an OpenStack environment, where
>> ceilometer was collecting domain statistics with libvirt. The domain
>> statistics reported by libvirt include the vCPU halted state, which 
>> in turn is retrieved with QMP query-cpus.
>> This causes two issues:
>> 1. Performance: on most architectures query-cpus needs to issue a KVM ioctl
>>    to find out whether a vCPU was halted. This is not the case for s390
>>    but query-cpus is always causing the vCPU to exit the VM.
>> 2. Semantics: on x86 and other architectures, halted is a highly transient
>>    state, which is likely to have already changed shortly after the state
>>    information has been retrieved. This is not the case for s390, where
>>    halted is an indication that the vCPU is stopped, meaning its not
>>    available to the guest operating system until it has been restarted.
>> The following patches help to alleviate the issues:
>> Patch 1/3:
>>   Adds architecture specific data to the QMP CpuInfo type, exposing
>>   the existing s390 cpu-state in QMP. The cpu-state is a representation
>>   more adequate than the ambiguous 'halted' condition.
>>   Changes since original v2:
>>   - fixed cpu-state usage in hw/intc/s390_flic.c, necessary because
>>     master was updated in the meantime
>>   - removed superfluous newline while printing cpu-state
> So this is adding s390x info in query-cpus (presumably for legacy
> callers?)...
Yeah, as long as we have the slow call, the fast data should be in there
as well.
> ...this is adding the new query-cpus-fast...
> ...and this is adding the s390x state to query-cpus-fast, right?

 Viktor Mihajlovski

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