[Libguestfs] [v2v PATCH] output/create_libvirt_xml: generate @check='none' CPU attribute

[Richard W.M. Jones]

On Wed, Jul 20, 2022 at 01:09:13PM +0200, Laszlo Ersek wrote:
> We currently don't generate any @check attribute for the /domain/cpu
> element, which causes the following libvirtd behavior
> <https://libvirt.org/formatdomain.html#cpu-model-and-topology>:
> 
> > Once the domain starts, libvirt will automatically change the check
> > attribute to the best supported value to ensure the virtual CPU does not
> > change when the domain is migrated to another host
> 
> Vera Wu reports that in practice, at least when the source CPU model is
> explicitly "qemu64", libvirtd sets @check='partial'. That's defined as:

I think my main confusion is why is the _source_ model "qemu64"?
Doesn't that imply the source hypervisor is qemu and so we shouldn't
use virt-v2v at all?  In fact from the BZ:

  # virt-v2v -i libvirt -ic qemu:///system esx6.7-rhel8.6-nvme-disk ...

(Vera is doing this for a good reason - she wants to test the -oo
compressed option, and using a local qemu guest is the quickest way.)

Anyway let's go with the assumption this is a bad idea, but for
testing purposes we often do this so we should try to make it work.

> > Libvirt will check the guest CPU specification before starting a domain
> 
> This is a problem: the default "qemu64" CPU model exposes the SVM CPU
> flag, and that's unsupportable on Intel hosts. SVM is the AMD counterpart
> of VT-x; IOW, the flag effectively advertizes AMD-specific nesting to
> guests.
> 
> With @check='partial', libvirt prevents the converted domain from starting
> on Intel hosts; but with @check='none',
> 
> > Libvirt does no checking and it is up to the hypervisor to refuse to
> > start the domain if it cannot provide the requested CPU. With QEMU this
> > means no checking is done at all since the default behavior of QEMU is
> > to emit warnings, but start the domain anyway.
> 
> We don't care about the migratability of the converted domain, so relax
> the libvirtd checks, by generating the @check='none' attribute.

For testing we don't, but in general -o libvirt conversions we might
care about migratability.

> Consider adding @check='none' in two cases:
> 
> (1) When the source domain specifies a CPU model.
> 
> Generating @check='none' in this case fixes the issue reported by Vera.
> 
> (2) When the source domain does not specify a CPU model, and the guest OS
> is assumed to work well with the default CPU model.
> 
> Generating @check='none' in this case is actually a no-op. Going from "no
> CPU element" to "<cpu check='none'/>" does not change how libvirtd
> augments the domain config. Namely,
> 
> (2.1) for x86 guests, we get
> 
> <cpu mode='custom' match='exact' check='none'>
>   <model fallback='forbid'>qemu64</model>
> </cpu>
> 
> either way,
> 
> (2.2) for aarch64 guests, we get
> 
> <cpu mode='custom' match='exact' check='none'>
>   <model fallback='forbid'>cortex-a15</model>
> </cpu>
> 
> either way.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2107503
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
>  output/create_libvirt_xml.ml | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/output/create_libvirt_xml.ml b/output/create_libvirt_xml.ml
> index 531a4f75bf3e..0343d3194268 100644
> --- a/output/create_libvirt_xml.ml
> +++ b/output/create_libvirt_xml.ml
> @@ -192,6 +192,7 @@ let create_libvirt_xml ?pool source inspect
>             List.push_back cpu_attrs ("mode", "host-passthrough");
>       | Some model ->
>           List.push_back cpu_attrs ("match", "minimum");
> +         List.push_back cpu_attrs ("check", "none");

So should we make this attribute conditional on the source CPU model
being qemu64, just so the change is minimal and we don't break
migratability for non testcases?

Rich.

>           (match source.s_cpu_vendor with
>            | None -> ()
>            | Some vendor ->
> -- 
> 2.19.1.3.g30247aa5d201
> 
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-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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