Re: [PATCH 1/2] hw/core: allow parameter=1 for SMP topology on any machine

Date: Mon, 13 May 2024 13:33:57 +0100 From: "Daniel P. Berrangé" <berrange(a)redhat.com&gt; Subject: [PATCH 1/2] hw/core: allow parameter=1 for SMP topology on any machine This effectively reverts commit 54c4ea8f3ae614054079395842128a856a73dbf9 Author: Zhao Liu <zhao1.liu(a)intel.com&gt; Date: Sat Mar 9 00:01:37 2024 +0800 hw/core/machine-smp: Deprecate unsupported "parameter=1" SMP configurations but is not done as a 'git revert' since the part of the changes to the file hw/core/machine-smp.c which add 'has_XXX' checks remain desirable. Furthermore, we have to tweak the subsequently added unit test to account for differing warning message. The rationale for the original deprecation was: "Currently, it was allowed for users to specify the unsupported topology parameter as "1". For example, x86 PC machine doesn't support drawer/book/cluster topology levels, but user could specify "-smp drawers=1,books=1,clusters=1". This is meaningless and confusing, so that the support for this kind of configurations is marked deprecated since 9.0." There are varying POVs on the topic of 'unsupported' topology levels. It is common to say that on a system without hyperthreading, that there is always 1 thread. Likewise when new CPUs introduced a concept of multiple "dies', it was reasonable to say that all historical CPUs before that implicitly had 1 'die'. Likewise for the more recently introduced 'modules' and 'clusters' parameter'. From this POV, it is valid to set 'parameter=1' on the -smp command line for any machine, only a value > 1 is strictly an error condition. It doesn't cause any functional difficulty for QEMU, because internally the QEMU code is itself assuming that all "unsupported" parameters implicitly have a value of '1'. At the libvirt level, we've allowed applications to set 'parameter=1' when configuring a guest, and pass that through to QEMU. Deprecating this creates extra difficulty for because there's no info exposed from QEMU about which machine types "support" which parameters. Thus, libvirt can't know whether it is valid to pass 'parameter=1' for a given machine type, or whether it will trigger deprecation messages. Since there's no apparent functional benefit to deleting this deprecated behaviour from QEMU, and it creates problems for consumers of QEMU, remove this deprecation. Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; --- docs/about/deprecated.rst | 14 ------- hw/core/machine-smp.c | 82 ++++++++++++------------------------- tests/unit/test-smp-parse.c | 8 ++-- 3 files changed, 30 insertions(+), 74 deletions(-) diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst index e22acb17f2..5b551b12a6 100644 --- a/docs/about/deprecated.rst +++ b/docs/about/deprecated.rst @@ -47,20 +47,6 @@ as short-form boolean values, and passed to plugins as ``arg_name=on``. However, short-form booleans are deprecated and full explicit ``arg_name=on`` form is preferred. -``-smp`` (Unsupported "parameter=1" SMP configurations) (since 9.0) -''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' - -Specified CPU topology parameters must be supported by the machine. - -In the SMP configuration, users should provide the CPU topology parameters that -are supported by the target machine. - -However, historically it was allowed for users to specify the unsupported -topology parameter as "1", which is meaningless. So support for this kind of -configurations (e.g. -smp drawers=1,books=1,clusters=1 for x86 PC machine) is -marked deprecated since 9.0, users have to ensure that all the topology members -described with -smp are supported by the target machine. - User-mode emulator command line arguments ----------------------------------------- diff --git a/hw/core/machine-smp.c b/hw/core/machine-smp.c index 2b93fa99c9..eb43caca9b 100644 --- a/hw/core/machine-smp.c +++ b/hw/core/machine-smp.c @@ -119,75 +119,45 @@ void machine_parse_smp_config(MachineState *ms, /* * If not supported by the machine, a topology parameter must be

- * omitted. + * not be set to a value greater than 1. */