[libvirt] [PATCH v2 06/11] docs: document cpu host-passthrough migration issue

[Jiri Denemark]

On Fri, Dec 09, 2016 at 14:38:35 -0500, Jason J. Herne wrote:
> Documents in formatdomain.html that when migrating a guest
> defined with the host-passthrough CPU model from a machine that
> is running on a newer CPU model than the destination machine's
> CPU model, it is very likely that the guest will crash upon
> arrival.
> 
> Signed-off-by: Jason J. Herne <jjherne at linux.vnet.ibm.com>
> ---
>  docs/formatdomain.html.in | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
> index 9218eec..b7c1e87 100644
> --- a/docs/formatdomain.html.in
> +++ b/docs/formatdomain.html.in
> @@ -1292,7 +1292,11 @@
>            understand. Though the downside of this mode is that the guest
>            environment cannot be reproduced on different hardware. Thus, if you
>            hit any bugs, you are on your own. Further details of that CPU can
> -          be changed using <code>feature</code> elements.</dd>
> +          be changed using <code>feature</code> elements. Migration of a guest
> +          using host-passthrough is dangerous if the source and destination
> +          hosts are not identical in both hardware and configuration. If such
> +          a migration is attempted then the guest will very likely hang or
> +          crash upon resuming execution on the destination host.</dd>
>          </dl>

I'd say "may" rather than "will very likely" :-) The likelihood of such
behavior depends on a lot of factors, such as the host architecture, the
way the hosts differ, etc. For example, on x86_64 such migration will
work just fine as long as the destination host is not worse than the
source host and even if it is, it depends on what the guest is doing.

ACK and it can be pushed right away, just let me know whether you agree
with the suggested change.

Jirka