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Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt

From: Anthony Liguori
Subject: Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt
Date: Sun, 11 Mar 2012 10:33:15 -0500
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On 03/11/2012 09:56 AM, Gleb Natapov wrote:
On Sun, Mar 11, 2012 at 09:12:58AM -0500, Anthony Liguori wrote:
-cpu best wouldn't solve this.  You need a read/write configuration
file where QEMU probes the available CPU and records it to be used
for the lifetime of the VM.
That what I thought too, but this shouldn't be the case (Avi's idea).
We need two things: 1) CPU model config should be per machine type.
2) QEMU should refuse to start if it cannot create cpu exactly as
specified by model config.

This would either mean:

A. pc-1.1 uses -cpu best with a fixed mask for 1.1

B. pc-1.1 hardcodes Westmere or some other family

(A) would imply a different CPU if you moved the machine from one system to another. I would think this would be very problematic from a user's perspective.

(B) would imply that we had to choose the least common denominator which is essentially what we do today with qemu64. If you want to just switch qemu64 to Conroe, I don't think that's a huge difference from what we have today.

It's a discussion about how we handle this up and down the stack.

The question is who should define and manage CPU compatibility.
Right now QEMU does to a certain degree, libvirt discards this and
does it's own thing, and VDSM/ovirt-engine assume that we're
providing something and has built a UI around it.
If we want QEMU to be usable without management layer then QEMU should
provide stable CPU models. Stable in a sense that qemu, kernel or CPU
upgrade does not change what guest sees.

We do this today by exposing -cpu qemu64 by default. If all you're advocating is doing -cpu Conroe by default, that's fine.

But I fail to see where this fits into the larger discussion here. The problem to solve is: I want to use the largest possible subset of CPU features available uniformly throughout my datacenter.

QEMU and libvirt have single node views so they cannot solve this problem on their own. Whether that subset is a generic Westmere-like processor that never existed IRL or a specific Westmere processor seems like a decision that should be made by the datacenter level manager with the node level view.

If I have a homogeneous environments of Xeon 7540, I would probably like to see a Xeon 7540 in my guest. Doesn't it make sense to enable the management tool to make this decision?


Anthony Liguori

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