Re: [PATCH manager/qemu-server 0/8] Add API and UI for custom CPU models

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

On Thu, Mar 26, 2026 at 03:54:53PM +0100, Fiona Ebner wrote:
> Am 12.03.26 um 9:40 AM schrieb Arthur Bied-Charreton:
> > This is picked up from an old series [0] by Stefan Reiter.
> > 
> > As of before this series, the only way to create custom CPU models is by
> > editing `/etc/pve/virtual-guest/cpu-models.conf` manually.
> > 
> > This can be cumbersome for a few reasons, e.g., due to the fact that flags
> > misconfigurations are only caught when starting the VM.
> > 
> > `cpu-flags` endpoint:
> > The `cpu-flags` endpoint previously returned a list of hardcoded flags,
> > which is both non-exhaustive (some flags I should be able to set are missing),
> > and partly incorrect (some flags my host(s) do not support set are returned).
> > This is limiting and can lead to misconfigurations. The updated endpoint
> > intersects all flags QEMU accepts as `-cpu` arguments with all flags the host
> > hardware/emulation actually supports. This way, if I am able to set a flag in
> > the UI, I can be confident that the VM will actually be able to start.
> > 
> > Custom CPU model CRUD functionality:
> > Expose CRUD endpoints and UI flow to interact with `cpu-models.conf`. For each
> > flag, show a list of the cluster nodes supporting it, and only expose flags that
> > at least one node supports to avoid misconfigurations. Filter flags by
> > acceleration type (KVM/TCG).
> > 
> > [0] https://lore.proxmox.com/pve-devel/20211028114150.3245864-1-s.reiter@proxmox.com/
> 
> With a pre-existing, manually added model:
> 
> [I] root@pve9a1 ~# cat /etc/pve/virtual-guest/cpu-models.conf
> cpu-model: nested-for-wsl
> 	flags +svm
> 	hidden 1
> 	hv-vendor-id AuthenticAMD
> 	reported-model EPYC
> 
> When opening it up in the UI, the editor seems to immediately detect a
> change, i.e. the OK button is clickable and when I click it, I get the
> following error:
> 
> Parameter verification failed. (400)
> flags: value does not match the regex pattern
> 
Thanks a lot for catching that!

> It seems to be because of the 'svm' flag not being in the flags grid.
Yes, just confirmed that.
> Since users might've already added such models manually, it would be
> nice if it would work. Maybe list the unknown flags in the grid without
> description?
Yes, I will display unknown flags in the grid with empty supported-on
fields. 

On this topic: I currently manually filter out occurrences of svm/vmx to 
replace them with nested-virt. Would it make sense to still display them
in the flags selector, even if they overlap with our abstraction?