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Bug 2051549 - optimize CPU flag probing QMP conversation
Summary: optimize CPU flag probing QMP conversation
Keywords:
Status: CLOSED MIGRATED
Alias: None
Product: Red Hat Enterprise Linux 9
Classification: Red Hat
Component: libvirt
Version: 9.0
Hardware: Unspecified
OS: Unspecified
unspecified
unspecified
Target Milestone: rc
: ---
Assignee: Jiri Denemark
QA Contact: Luyao Huang
URL:
Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2022-02-07 13:26 UTC by Peter Krempa
Modified: 2023-09-22 12:19 UTC (History)
4 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
Environment:
Last Closed: 2023-09-22 12:19:56 UTC
Type: Bug
Target Upstream Version:
Embargoed:
pm-rhel: mirror+

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System ID Private Priority Status Summary Last Updated
Red Hat Issue Tracker   RHEL-7038 0 None Migrated None 2023-09-22 12:19:49 UTC
Red Hat Issue Tracker RHELPLAN-111348 0 None None None 2022-02-07 13:29:14 UTC

Description Peter Krempa 2022-02-07 13:26:44 UTC 
Description of problem:
Libvirt uses a very inefficient way to probe qemu for CPU feature flags, which is done by using ~400 QMP calls and responses to fetch all the required information as each flag is queried one-by-one. This is due to the lack of a better interface in qemu.

This takes a significant amount of time during the startup of the VM:

2022-02-07 11:41:50.811+0000: 1758242: info : qemuMonitorIOWrite:402 : QEMU_MONITOR_IO_WRITE: mon=0x7fff9c1d2a20 buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"realized"},"id":"libvirt-8"}
 len=115 ret=115 errno=0
 
[...]

2022-02-07 11:41:50.950+0000: 1758242: info : qemuMonitorIOWrite:402 : QEMU_MONITOR_IO_WRITE: mon=0x7fff9c1d2a20 buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"},"id":"libvirt-395"}
 len=129 ret=129 errno=0

In my example case it's 0.14s just to probe the features.

Libvirt should work with qemu to provide a better interface where we can fetch multiple flags at once.
Comment 1 Andrew Jones 2022-02-09 11:11:05 UTC 
Is the QMP CPU model expansion query[*] not being used, or is it insufficient?

https://qemu-project.gitlab.io/qemu/interop/qemu-qmp-ref.html#qapidoc-2595
Comment 2 Jiri Denemark 2022-02-09 11:47:08 UTC 
This is a bit different thing. We use expansion before starting a domain to be
able to construct a QEMU command line corresponding to our XML, but once QEMU
is up and ready to start vCPUs, we want to see exactly what features were
enabled and disabled for the guest. That is the current state of the vCPUs.
Historically we used CPUID bits, which was quite easy and required very little
QMP commands, but we can't do that anymore since some features started to be
advertised in MSRs. Thus we need to check the features by their names in QOM
and while listing all of them is just one QMP command for all, getting their
values is one QMP command per CPU features, which means hundreds of commands
just to probe what features are actually visible to a guest.
Comment 7 RHEL Program Management 2023-09-22 12:18:53 UTC 
Issue migration from Bugzilla to Jira is in process at this time. This will be the last message in Jira copied from the Bugzilla bug.
Comment 8 RHEL Program Management 2023-09-22 12:19:56 UTC 
This BZ has been automatically migrated to the issues.redhat.com Red Hat Issue Tracker. All future work related to this report will be managed there.

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