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Re: [libvirt] Qemu, libvirt, and CPU models



On Tue, Mar 06, 2012 at 03:27:53PM -0300, Eduardo Habkost wrote:
> Hi,
> 
> Sorry for the long message, but I didn't find a way to summarize the
> questions and issues and make it shorter.
> 
> For people who don't know me: I have started to work recently on the
> Qemu CPU model code. I have been looking at how things work on
> libvirt+Qemu today w.r.t. CPU models, and I have some points I would
> like to understand better and see if they can be improved.
> 
> I have two main points I would like to understand/discuss:
> 
> 1) The relationship between libvirt's cpu_map.xml and the Qemu CPU model
>    definitions.

We have several areas of code in which we use CPU definitions

 - Reporting the host CPU definition (virsh capabilities)
 - Calculating host CPU compatibility / baseline definitions
 - Checking guest / host CPU compatibility
 - Configuring the guest CPU definition

libvirt targets multiple platforms, and our CPU handling code is designed
to be common & sharable across all the libvirt drivers, VMWare, Xen, KVM,
LXC, etc. Obviously for container based virt, only the host side of things
is relevant.

The libvirt CPU XML definition consists of

 - Model name
 - Vendor name
 - zero or more feature flags added/removed.

A model name is basically just an alias for a bunch of feature flags,
so that the CPU XML definitions are a) reasonably short b) have
some sensible default baselines.

The cpu_map.xml is the database of the CPU models that libvirt
supports. We use this database to transform the CPU definition
from the guest XML, into the hypervisor's own format.

As luck would have it, the cpu_map.xml file contents match what
QEMU has. This need not be the case though. If there is a model
in the libvirt cpu_map.xml that QEMU doesn't know, we'll just
pick the nearest matching QEMU cpu model & specify the fature
flags to compensate. We could go one step further and just write
out a cpu.conf file that we load in QEMU with -loadconfig.

On Xen we would use the cpu_map.xml to generate the CPUID
masks that Xen expects. Similarly for VMWare.

> 2) How we could properly allow CPU models to be changed without breaking
>    existing virtual machines?

What is the scope of changes expected to CPU models ?

> 1) Qemu and cpu_map.xml
> 
> I would like to understand how cpu_map.xml is supposed to be used, and
> how it is supposed to interact with the CPU model definitions provided
> by Qemu. More precisely:
> 
> 1.1) Do we want to eliminate the duplication between the Qemu CPU
>   definitions and cpu_map.xml?

It isn't possible for us to the libvirt cpu_map.xml, since we
need that across all our hypervisor targets.

> 1.1.1) If we want to eliminate the duplication, how can we accomplish
>   that? What interfaces you miss, that Qemu could provide?
> 
> 1.1.2) If the duplication has a purpose and you want to keep
>   cpu_map.xml, then:
>   - First, I would like to understand why libvirt needs cpu_map.xml? Is
>     it part of the "public" interface of libvirt, or is it just an
>     internal file where libvirt stores non-user-visible data?
>   - How can we make sure there is no confusion between libvirt and Qemu
>     about the CPU models? For example, what if cpu_map.xml says model
>     'Moo' has the flag 'foo' enabled, but Qemu disagrees? How do we
>     guarantee that libvirt gets exactly what it expects from Qemu when
>     it asks for a CPU model? We have "-cpu ?dump" today, but it's not
>     the better interface we could have. Do you miss something in special
>     in the Qemu<->libvirt interface, to help on that?
> 
> 1.2) About the probing of available features on the host system: Qemu
>   has code specialized to query KVM about the available features, and to
>   check what can be enabled and what can't be enabled in a VM. On many
>   cases, the available features match exactly what is returned by the
>   CPUID instruction on the host system, but there are some
>   exceptions:
>   - Some features can be enabled even when the host CPU doesn't support
>     it (because they are completely emulated by KVM, e.g. x2apic).
>   - On many other cases, the feature may be available but we have to
>     check if Qemu+KVM are really able to expose it to the guest (many
>     features work this way, as many depend on specific support by the
>     KVM kernel module and/or Qemu).
>   
>   I suppose libvirt does want to check which flags can be enabled in a
>   VM, as it already have checks for host CPU features (e.g.
>   src/cpu/cpu_x86.c:x86Compute()). But I also suppose that libvirt
>   doesn't want to duplicate the KVM feature probing code present on
>   Qemu, and in this case we could have an interface where libvirt could
>   query for the actually-available CPU features. Would it be useful for
>   libvirt? What's the best way to expose this interface?
> 
> 1.3) Some features are not plain CPU feature bits: e.g. level=X can be
>   set in "-cpu" argument, and other features are enabled/disabled by
>   exposing specific CPUID leafs and not just a feature bit (e.g. PMU
>   CPUID leaf support). I suppose libvirt wants to be able to probe for
>   those features too, and be able to enable/disable them, right?


The libvirt CPU definition does not currently store info about the
level, family, model, stepping, xlevel or model_id items. We really
ought to fix this, so that libvirt does have that info. Then we'd
be able to write out a QEMU config that fully specified the exact
model.

> 2) How to change an existing model and keep existing VMs working?
> 
> Sometimes we have to update a CPU model definition because of some bug.
> Eamples:
> 
> - The CPU models Conroe, Penrym and Nehalem, have level=2 set. This
>   works most times, but it breaks CPU core/thread topology enumeration.
>   We have to change those CPU models to use level=4 to fix the bug.

This is an example of why libvirt needs to represent the level/family
etc in its CPU definition. That way, when a guest is first created,
the XML will save the CPU model, feature flags, level, family, etc
it is created with. Should the level be changed later, existing guests
would then not be affected, only new guests would get the level=4

> - This can happen with plain CPU feature bits, too, not just "level":
>   sometimes real-world CPU models have a feature that is not supported
>   by Qemu+KVM yet, but when the kernel and Qemu finally starts to
>   support it, we may want to enable it on existing CPU models. Sometimes
>   a model simply has the wrong set of feature bits, and we have to fix
>   it to have the right set of features.

> 2.3) How all this will interact with cpu_map.xml? Right now there's the
>   assumption that the CPU model definitions are immutable, right?
> 
> 2.4) How do you think libvirt would expose this "CPU model version"
>   to the user? Should it just expose the unversioned CPU models to the
>   user, and let Qemu or libvirt choose the right version based on
>   machine-type?  Should it expose only the versioned CPU models (because
>   they are immutable) and never expose the unversioned aliases? Should
>   it expose the unversioned alias, but change the Domain XML definition
>   automatically to the versioned immutable one (like it happens with
>   machine-type)?

We should only expose unversioned CPU models, but then record the
precise details of the current version in the guest XML.

> I don't plan to interfere on the libvirt interface design, but I suppose
> that libvirt design assumptions will be impacted by the solution we
> choose on Qemu. For example: right now libvirt seems to assume that CPU
> models are immutable. Are you going to keep this assumption in the
> libvirt interfaces? Because I am already willing to break this
> assumption on Qemu, although I would like to cooperate with libvirt and
> not break any requirements/assumptions without warning.


Daniel
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