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Re: [Qemu-devel] cpuid problem in upstream qemu with kvm

From: Dor Laor
Subject: Re: [Qemu-devel] cpuid problem in upstream qemu with kvm
Date: Thu, 07 Jan 2010 11:11:18 +0200
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On 01/07/2010 10:18 AM, Avi Kivity wrote:
On 01/07/2010 10:03 AM, Dor Laor wrote:

We can debate about the exact name/model to represent the Nehalem
family, I don't have an issue with that and actually Intel and Amd
should define it.

AMD and Intel already defined their names (in cat /proc/cpuinfo). They
don't define families, the whole idea is to segment the market.

The idea here is to minimize the number of models we should have the following range for Intel for example:
  pentium3 - merom -  penry - Nehalem - host - kvm/qemu64
So we're supplying wide range of cpus, p3 for maximum flexibility and migration, nehalem for performance and migration, host for maximum performance and qemu/kvm64 for custom maid.

There are two main motivations behind the above approach:
1. Sound guest cpu definition.
Using a predefined model should automatically set all the relevant
vendor/stepping/cpuid flags/cache sizes/etc.
We just can let every management application deal with it. It breaks
guest OS/apps. For instance there are MSI support in windows guest
relay on the stepping.

2. Simplifying end user and mgmt tools.
qemu/kvm have the best knowledge about these low levels. If we push
it up in the stack, eventually it reaches the user. The end user,
not a 'qemu-devel user' which is actually far better from the
average user.

This means that such users will have to know what is popcount and
whether or not to limit migration on one host by adding sse4.2 or

This is exactly what vmware are doing:
- Intel CPUs :

- AMD CPUs :

They don't have to deal with different qemu and kvm versions.

Both our customers - the end users. It's not their problem.
IMO what's missing today is a safe and sound cpu emulation that is simply and friendly to represent. qemu64,+popcount is not simple for the end user. There is no reason to through it on higher level mgmt.

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