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Re: [Qemu-devel] [qemu-s390x] [PATCH 1/3] qmp: expose s390-specific CPU

From: Eric Blake
Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH 1/3] qmp: expose s390-specific CPU info
Date: Tue, 13 Feb 2018 09:10:22 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 02/13/2018 06:20 AM, Viktor Mihajlovski wrote:
We're adding the same information to query-cpus-fast. Why do we
need to duplicate this into query-cpus? Do you plan to keep using
query-cpus? If yes, why?

Wonder if we could simply pass a flag to query-cpus "fast=true", that
makes it behave differently. (either not indicate the critical values or
simply provide dummy values - e.g. simply halted=false)

That was one of the ideas in the earlier stages of this discussion (and
I was inclined to go that way initially). The major drawback of this
approach that the slow call is hard to deprecate (OK, one could change
the default to fast=true over time). It's easier to deprecate the entire
query-cpus function. The other issue, maybe not as bad, is that one has
to deal with fields that are suddenly optional or obsolete in way not
confusing every one.
Bottom line is that I'm convinced it's better to have both APIs and to
deprecate the slow one over time. But I have to confess I'm not familiar
with QAPI deprecation rules, maybe Eric can shed some light on this...

You are correct that it is easier to have two commands if we plan for one to completely disappear (after a proper deprecation period, where it is well-documented for a couple of releases that we plan on removing the older command). The alternative of adding a bool that controls whether the painful fields are omitted, is partially introspecitble (you can learn whether the new bool exists, in which case you use it for the new behavior), but later changing the default value of that bool over time is not (as we don't yet have a way to introspect default values - maybe we should add that someday, but we're not there yet). But right now, it is easy to introspect the addition of a new command (if it exists, use it) and a later disappearance of a command. So I'm in agreement with your approach of a new command.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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