Re: [PATCH v2 00/10] Generate x86 cpu features

[Igor Mammedov <imammedo@redhat.com>]

On Fri,  8 Sep 2023 14:45:24 +0200
Tim Wiederhake <twiederh@redhat.com> wrote:

> Synchronizing the list of cpu features and models with qemu is a recurring
> task in libvirt. For x86, this is done by reading qom-list-properties for
> max-x86_64-cpu and manually filtering out everthing that does not look like
> a feature name, as well as parsing target/i386/cpu.c for cpu models.

modulo fixing typos/name conflicts in 1st 3 patches,

I don't think it's a great idea for libvirt (or any other user) to parse
QEMU source (whether it's C code or yaml) or other way around for users
to influence QEMU internals.

QEMU does provides QMP interface for introspection of CPU models and it
should be used as a mean to discover new CPU models as well as new
feature names. If something is missing there we should fix it up
to make it usable for libvirt.

> This is a flawed, tedious and error-prone procedure. Ideally, qemu
> and libvirt would query a common source for cpu feature and model
> related information. Meanwhile, converting this information into an easier
> to parse format would help libvirt a lot.
> 
> This patch series converts the cpu feature information present in
> target/i386/cpu.c (`feature_word_info`) into a yaml file and adds a
> script to generate the c code from this data.

 
> A patch set to convert the cpu model data (`builtin_x86_defs`) in the
> same way will follow.

while theoretically you can parse feature_word_info with some sort of reliability,
you can't do that reliably with CPU model definitions in builtin_x86_defs
as the later is just a skeleton. Actual CPU models definitions are
a function of used machine type, accelerator and host kernel. 

> 
> v1: https://lists.nongnu.org/archive/html/qemu-devel/2023-08/msg02005.html
> 
> Changes since v1:
> * Incorporated changes from
>   https://lists.nongnu.org/archive/html/qemu-devel/2023-08/msg04241.html.
> * Changed data format from xml to yaml, as proposed in
>   https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg01033.html.
>   Using json has some drawbacks, see
>   https://lists.nongnu.org/archive/html/qemu-devel/2023-08/msg03384.html.
> * Rebased on top of current master. Features added in the meantime:
>   amx-complex and vmx-enable-user-wait-pause
> * Split up the reformatting of feature_word_info.c.inc to make it easier
>   to review. These patches could be squashed together before merging.
> 
> Tim Wiederhake (10):
>   target/i386: Add missing feature names in FEAT_VMX_EPT_VPID_CAPS
>   target/i386: Fix feature names in FEAT_VMX_EPT_VPID_CAPS
>   target/i386: Fix duplicated feature name in FEAT_KVM
>   target/i386: Split out feature_word_info
>   target/i386: Translate feature_word_info to yaml
>   target/i386: Format feature_word_info.c.inc: Remove comments
>   target/i386: Format feature_word_info.c.inc: feat_names
>   target/i386: Format feature_word_info.c.inc: Unfold cpuid member
>   target/i386: Format feature_word_info.c.inc: Whitespaces and trailing
>     commas
>   target/i386: Autogenerate feature_word_info.c.inc
> 
>  target/i386/cpu.c                   |  677 +-----------------
>  target/i386/feature_word_info.c.inc | 1030 +++++++++++++++++++++++++++
>  target/i386/feature_word_info.py    |   62 ++
>  target/i386/feature_word_info.yaml  |  697 ++++++++++++++++++
>  4 files changed, 1790 insertions(+), 676 deletions(-)
>  create mode 100644 target/i386/feature_word_info.c.inc
>  create mode 100755 target/i386/feature_word_info.py
>  create mode 100644 target/i386/feature_word_info.yaml
>