Re: [PATCH v3 2/3] migration/docs: How to migrate when hosts have different features

[Peter Xu]

On Mon, May 15, 2023 at 10:32:00AM +0200, Juan Quintela wrote:
> Sometimes devices have different features depending of things outside
> of qemu.  For instance the kernel.  Document how to handle that cases.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> 
> ---
> 
> If you have some example to put here, I am all ears.  I guess that
> virtio-* with some features that are on qemu but not on all kernel
> would do the trick, but I am not a virtio guru myself.  Patches
> welcome.
> ---
>  docs/devel/migration.rst | 93 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 93 insertions(+)
> 
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> index b4c4f3ec35..95e797ee60 100644
> --- a/docs/devel/migration.rst
> +++ b/docs/devel/migration.rst
> @@ -357,6 +357,99 @@ machine types to have the right value: ::
>           ...
>       };
>  
> +A device with diferent features on both sides
> +---------------------------------------------
> +
> +Let's assume that we are using the same QEMU binary on both sides,
> +just to make the things easier.  But we have a device that has
> +different features on both sides of the migration.  That can be
> +because the devices are different, because the kernel driver of both
> +devices have different features, whatever.
> +
> +How can we get this to work with migration.  The way to do that is
> +"theoretically" easy.  You have to get the features that the device
> +has in the source of the migration.  The features that the device has
> +on the target of the migration, you get the intersection of the
> +features of both sides, and that is the way that you should launch
> +qemu.
> +
> +Notice that this is not completely related to qemu.  The most
> +important thing here is that this should be handle by the managing
> +application that launches qemu.  If qemu is configured correctly, the
> +migration will suceeed.
> +
> +Once that we have defined that, doing this is complicated.  Almost all
> +devices are bad at being able to be launched with only some features
> +enabled.  With one big exception: cpus.
> +
> +You can read the documentation for QEMU x86 cpu models here:
> +
> +https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html
> +
> +See when they talk about migration they recommend that one chooses the
> +newest cpu model that is supported for all cpus.
> +
> +Let's say that we have:
> +
> +Host A:
> +
> +Device X has the feature Y
> +
> +Host B:
> +
> +Device X has not the feature Y
> +
> +If we try to migrate without any care from host A to host B, it will
> +fail because when migration tries to load the feature Y on
> +destination, it will find that the hardware is not there.
> +
> +Doing this would be the equivalent of doing with cpus:
> +
> +Host A:
> +
> +$ qemu-system-x86_64 -cpu host
> +
> +Host B:
> +
> +$ qemu-system-x86_64 -cpu host
> +
> +When both hosts have different cpu features this is waranteed to fail.
> +Especially if Host B has less features than host A.  If host A has
> +less features than host B, sometimes it works.  Important word of last
> +sentence is "sometimes".
> +
> +So, forgetting about cpu models and continuing with the -cpu host
> +example, let's see that the differences of the cpus is that Host A and
> +B have the following features:
> +
> +Features:   'pcid'  'stibp' 'taa-no'
> +Host A:        X       X
> +Host B:                        X
> +
> +And we want to migrate between them, the way configure both qemu cpu
> +will be:
> +
> +Host A:
> +
> +$ qemu-system-x86_64 -cpu host,pcid=off,stibp=off
> +
> +Host B:
> +
> +$ qemu-system-x86_64 -cpu host,taa-no=off

Since we're using cpu as example, shall we at least mention at the end that
we don't suggest using -cpu host if migration is needed?

> +
> +And you would be able to migrate between them.  It is responsability
> +of the management application or of the user to make sure that the
> +configuration is correct.  QEMU don't know how to look at this kind of
> +features in general.
> +
> +Other devices have worse control about individual features.  If they
> +want to be able to migrate between hosts that show different features,
> +the device needs a way to configure which ones it is going to use.
> +
> +In this section we have considered that we are using the same QEMU
> +binary in both sides of the migration.  If we use different QEMU
> +versions process, then we need to have into account all other
> +differences and the examples become even more complicated.

Mostly good to me.  What I worry is how much help this will bring to
developers - I'd assume developers working on these will be aware of this.
But I guess it's always good to have any documentation than nothing.

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu