[Libguestfs] [v2v PATCH v2] output/create_libvirt_xml: relax VCPU feature checking for "qemu64"

[Richard W.M. Jones]

On Fri, Jul 22, 2022 at 09:36:27AM +0200, Laszlo Ersek wrote:
> When the source domain doesn't specify a VCPU model ("s_cpu_model" is
> None), and the guest OS is assumed to work with the default VCPU model
> ("gcaps_default_cpu" is true), we don't output any <cpu> element. In that
> case, libvirtd augments the domain config with:
> 
>   [1] <cpu mode='custom' match='exact' check='none'>
>         <model fallback='forbid'>qemu64</model>
>       </cpu>
> 
> where the @check='none' attribute ensures that the converted domain will
> be launched, for example, on an Intel host, despite the "qemu64" VCPU
> model containing AMD-only feature flags such as "svm".
> 
> However, if the source domain explicitly specifies the "qemu64" model
> (mostly seen with "-i libvirt -ic qemu://..."), we presently output
> 
>   [2] <cpu match='minimum'>
>         <model fallback='allow'>qemu64</model>
>       </cpu>
> 
> which libvirtd completes as
> 
>   [3] <cpu mode='custom' match='minimum' check='partial'>
>         <model fallback='allow'>qemu64</model>
>       </cpu>
> 
> In [3], cpu/@match='minimum' and cpu/model/@fallback='allow' are both
> laxer than @match='exact' and @fallback='forbid', respectively, in [1].
> 
> However, cpu/@check='partial' in [3] is stricter than @check='none' in
> [1]; it causes libvirtd to catch the "svm" feature flag on an Intel host,
> and prevents the converted domain from starting.
> 
> The "qemu64" VCPU model is supposed to run on every possible host
> <https://gitlab.com/qemu-project/qemu/-/blob/master/docs/system/cpu-models-x86.rst.inc>,
> therefore make an exception for the explicitly specified "qemu64" VCPU
> model, and generate the @check='none' attribute.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2107503
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
> 
> Notes:
>     v2:
>     - make the attribute dependent on "qemu64" [Rich]
>     - retrofit the commit message
>     
>     v1:
>     - https://listman.redhat.com/archives/libguestfs/2022-July/029504.html
> 
>  output/create_libvirt_xml.ml | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/output/create_libvirt_xml.ml b/output/create_libvirt_xml.ml
> index 531a4f75bf3e..bd01304df674 100644
> --- a/output/create_libvirt_xml.ml
> +++ b/output/create_libvirt_xml.ml
> @@ -192,6 +192,8 @@ let create_libvirt_xml ?pool source inspect
>             List.push_back cpu_attrs ("mode", "host-passthrough");
>       | Some model ->
>           List.push_back cpu_attrs ("match", "minimum");
> +         if model = "qemu64" then
> +           List.push_back cpu_attrs ("check", "none");
>           (match source.s_cpu_vendor with
>            | None -> ()
>            | Some vendor ->

ACK - but see also:
https://listman.redhat.com/archives/libguestfs/2022-July/029520.html

Rich.

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