Re: [PATCH v2] microvm: do not use the lastest cpu version

[Zhao Liu <zhao1.liu@intel.com>]

On Thu, Feb 20, 2025 at 12:23:26PM +0530, Ani Sinha wrote:
> Date: Thu, 20 Feb 2025 12:23:26 +0530
> From: Ani Sinha <anisinha@redhat.com>
> Subject: [PATCH v2] microvm: do not use the lastest cpu version
> X-Mailer: git-send-email 2.45.2
> 
> commit 0788a56bd1ae3 ("i386: Make unversioned CPU models be aliases")
> introduced 'default_cpu_version' for PCMachineClass. This created three
> categories of CPU models:
>  - Most unversioned CPU models would use version 1 by default.
>  - For machines 4.0.1 and older that do not support cpu model aliases, a
>    special default_cpu_version value of CPU_VERSION_LEGACY is used.
>  - It was thought that future machines would use the latest value of cpu
>    versions corresponding to default_cpu_version value of
>    CPU_VERSION_LATEST [1].
> 
> All pc machines still use the default cpu version of 1 for
> unversioned cpu models. CPU_VERSION_LATEST is a moving target and
> changes with time. Therefore, if machines use CPU_VERSION_LATEST, it would
> mean that over a period of time, for the same versioned machine type,
> the cpu version would be different depending on what the latest was at that
> time. This would break guests even when they use a constant specific
> versioned machine type.
> Additionally, microvm machines are not versioned anyway and therefore
> there is no requirement to use the latest cpu model by default.
> Let microvms use the non-versioned cpu model and remove all references
> to CPU_VERSION_LATEST as there are no other users (nor we anticipate
> future consumers of CPU_VERSION_LATEST).
> 
> Those users who need spefific cpu versions can use explicit version in
> the QEMU command line to select the specific cpu version desired.
> 
> CI pipline does not break with this change.
> 
> 1) See commit dcafd1ef0af227 ("i386: Register versioned CPU models")
> 
> CC: imammedo@redhat.com
> CC: zhao1.liu@intel.com
> Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> Reviewed-by: Sergio Lopez <slp@redhat.com>
> Signed-off-by: Ani Sinha <anisinha@redhat.com>
> ---
>  hw/i386/microvm.c |  2 +-
>  target/i386/cpu.c | 15 ---------------
>  target/i386/cpu.h |  4 ----
>  3 files changed, 1 insertion(+), 20 deletions(-)
> 
> changelog:
> v2: tags added, more explanation in the commit log.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>