Re: [libvirt] Cpu Modeling
Hi!
On Thu, Sep 22, 2016 at 02:47:36PM -0400, Jason J. Herne wrote:
Hi Jiri & Eduardo,
You might remember a discussion with David Hildenbrand of IBM on the Qemu
mailing list regarding a new Qemu<->Libvirt interface for cpu modeling. I am
picking up this work from David and I wanted to confirm that we are still on
the
same page as to the direction of that interface.
I am, but I would like to hear Jiri's feedback in case there's
anything that is still lacking in the new interface.
Additionanl comments below:
For your reference:
https://www.redhat.com/archives/libvir-list/2016-June/thread.html#01413
https://lists.gnu.org/archive/html/qemu-devel/2016-09/threads.html#00489
The first link is to the discussion you were directly involved in. The
second
link is to the final patch set posted to qemu-devel. The cover letter gives
a
good overview of the interface added to Qemu and the proposed use-case for
Libvirt to use this new cpu modeling support. I'll paste in the most
relevant
section for your convenience:
--------------------------------Libvirt usecase----------------------------
Testing for runability:
- Simply try to start QEMU with KVM, compat machine, CPU model
The main problem here is for libvirt to get machine-friendly
error information from QEMU in case the VM fails to start. But if
libvirt avoids starting QEMU based on previously-collected
runnability information, we should be OK.
- Could be done using query-cpu-model-comparison in the future.
I sent a series adding runnability information to
query-cpu-definitions as well.
Identifying host model, e.g. "virsh capabilities"
- query-cpu-model-expansion on "host" with "-M none --enable-kvm"
Correct.
<cpu mode='host-model'>:
- simply copy the identified host model
<cpu mode='host-passthrough'>:
- "-cpu host"
Correct.
"virsh cpu-baseline":
- query-cpu-model-baseline on two models with "-M none"
"virsh cpu-compare":
- query-cpu-model-comparison on two models with "-M none"
Correct.
There might be some cenarios where libvirt wants to convert another CPU
model to a static variant, this can be done using query-cpu-model-expansion
---------------------------------------------------------------------------
Now that I've hopefully refreshed your memory :) I just want to make sure
that
you are still on-board with this plan. I realize that this new approach does
things differently than Libvirt does today for other platforms. Especially
x86_64. The big differences are as follows:
1. We will invoke qemu to gather the host cpu data used for virsh
capabilities.
Today this data seems to be collected directly from the host hardware for
most
(all?) architectures.
I believe libvirt developers want to change this, because the
current approach is wrong for x86 as well.
2. virsh cpu-models {arch} will also use a Qemu invocation to gather cpu
model
data.
I believe libvirt can collect this data once, and cache it (like
it does today).
3. Most architectures seem to use a "map" (xml file containing cpu model
data
that ships with Libvirt) to satisfy #1 and #2 above. Our new method does not
use
this map as it gets all of the data directly from Qemu.
I believe the goal is to make cpu_map.xml unnecessary on x86 as
well.
4. virsh cpu-baseline and cpu-compare will now invoke qemu directly as well.
Note: I'm not sure exactly how much all of this will apply just to s390 with
other architectures moving over to the new interface if/when they want to.
Or if
we will want to change all architectures to this new interface at the same
time.
Any guidance?
I am not sure about this part, because I don't know what are the
existing use cases of cpu-baseline and cpu-compare. But I believe
this shouldn't be a problem as long as the function is not called
very often and/or results are cached by libvirt.
--
Eduardo