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Re: [PATCH v15 09/11] machine: adding s390 topology to query-cpu-fast

From: Thomas Huth
Subject: Re: [PATCH v15 09/11] machine: adding s390 topology to query-cpu-fast
Date: Tue, 7 Feb 2023 11:10:41 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0

On 06/02/2023 15.50, Daniel P. Berrangé wrote:
On Mon, Feb 06, 2023 at 02:09:57PM +0100, Thomas Huth wrote:
On 06/02/2023 13.49, Daniel P. Berrangé wrote:
On Mon, Feb 06, 2023 at 01:41:44PM +0100, Thomas Huth wrote:
On 01/02/2023 14.20, Pierre Morel wrote:
S390x provides two more topology containers above the sockets,
books and drawers.

Let's add these CPU attributes to the QAPI command query-cpu-fast.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
    qapi/machine.json          | 13 ++++++++++---
    hw/core/machine-qmp-cmds.c |  2 ++
    2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/qapi/machine.json b/qapi/machine.json
index 3036117059..e36c39e258 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -53,11 +53,18 @@
    # Additional information about a virtual S390 CPU
-# @cpu-state: the virtual CPU's state
+# @cpu-state: the virtual CPU's state (since 2.12)
+# @dedicated: the virtual CPU's dedication (since 8.0)
+# @polarity: the virtual CPU's polarity (since 8.0)
    # Since: 2.12
-{ 'struct': 'CpuInfoS390', 'data': { 'cpu-state': 'CpuS390State' } }
+{ 'struct': 'CpuInfoS390',
+    'data': { 'cpu-state': 'CpuS390State',
+              'dedicated': 'bool',
+              'polarity': 'int'

I think it would also be better to mark the new fields as optional and only
return them if the guest has the topology enabled, to avoid confusing
clients that were written before this change.

FWIW, I would say that the general expectation of QMP clients is that
they must *always* expect new fields to appear in dicts that are
returned in QMP replies. We add new fields at will on a frequent basis.

Did we change our policy here? I slightly remember I've been told
differently in the past ... but I can't recall where this was, it's
certainly been a while.

So a question to the QAPI maintainers: What's the preferred handling for new
fields nowadays in such situations?

I think you're mixing it up with policy for adding new fields to *input*
parameters. You can't add a new mandatory field to input params of a
command because no existing client will be sending that. Only optional
params are viable, without a deprecation cycle. For output parameters
such as this case, there's no compatibilty problem.

Ah, right, I likely mixed it up with that. Thanks for the clarification!


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