Re: [RFC qemu/qemu-server 0/2] qemu: dump and ship CPU model expansions

[Arthur Bied-Charreton <a.bied-charreton@proxmox.com>]

On Thu, Jul 30, 2026 at 04:15:08PM +0200, Fiona Ebner wrote:
> Sending this, so Arthur and Erik can look into it for issue #7841.
> 
> Having the CPU model expansions readily available makes it possible to
> be precise when checking for the presence of certain CPU flags or
> properties. For example, the pdpe1gb flag is relevant since OVMF will
> limit the phys-bits to 40 without that. Starting a QEMU process to
> query this via QMP is too costly in many cases, so qemu-server is
> currently limited to use heuristics.
> 
> Having the flags can also be useful for the custom CPU model editor to
> show what the default value for a given flag and model actually is.
> 
> The one for x86_64 is rather large with 1.65 MiB, but it can be loaded
> on first use and then cached by the CPUConfig.pm module.
> 
> It's important to be aware that some features/flags are gated behind
> others, most prominently, the nested virtualization ones.
> 
Thanks for sending this! Wouldn't it make more sense to build this
directly in such a way that we can query model expansions dynamically
though? As far as I can tell, gated flags mean that, as soon as a CPU 
config has one flag set/unset (which I think is a pretty common case),
we cannot rely on the static file anymore. There might be a way to find
out which features gate others, but this seems like a workaround that 
could be avoided entirely with dynamic queriability.

What do you think?
> pve-qemu:
> 
> Fiona Ebner (1):
>   d/rules: dump and ship CPU model expansions
> 
>  debian/cpu-model-expansions-aarch64.json |   220 +
>  debian/cpu-model-expansions-x86_64.json  | 51268 +++++++++++++++++++++
>  debian/dump-cpu-model-expansions.pl      |    66 +
>  debian/rules                             |    10 +
>  4 files changed, 51564 insertions(+)
>  create mode 100644 debian/cpu-model-expansions-aarch64.json
>  create mode 100644 debian/cpu-model-expansions-x86_64.json
>  create mode 100755 debian/dump-cpu-model-expansions.pl
> 
> 
> qemu-server:
> 
> Fiona Ebner (1):
>   cpu flags: introduce get_cpu_model_expansions() helper
> 
>  src/PVE/QemuServer/CPUFlags.pm | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> 
> Summary over all repositories:
>   5 files changed, 51591 insertions(+), 0 deletions(-)
> 
> -- 
> Generated by git-murpp 0.5.0
> 
> 
> 
>