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Re: [Qemu-devel] cpuid problem in upstream qemu with kvm
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] cpuid problem in upstream qemu with kvm |
Date: |
Mon, 14 Dec 2009 23:10:25 +0200 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Mon, Dec 14, 2009 at 02:54:49PM -0600, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>> On Mon, Dec 14, 2009 at 02:18:33PM -0600, Anthony Liguori wrote:
>>
>>> Michael S. Tsirkin wrote:
>>>
>>>> This might help 32 bit guests, but not guests with 64 bit
>>>> kernel and 32 bit userspace (my case) because all 64 bit
>>>> CPUs advertise syscall bit in cpuid. Thus 64 bit guests
>>>> do not seem to even bother checking this bit:
>>>> AMD + 64 bit -> syscall.
>>>>
>>> Okay, I don't see a great option other than migrating the vendor_id string.
>>>
>>
>> This won't help with kernels <2.6.32 though. I guess we can switch
>> default vendor to Intel, this likely has some other side effects.
>>
>
> That's a kernel bug. If we think it effects a lot of users, we should
> introduce a CAP such that we can detect this in userspace and fail
> gracefully.
Not emulating feature host CPU does not have is a kernel bug?
Okay ...
Yes, almost no one runs 2.6.32 yet.
>>> Otherwise, cross vendor migration will not work by default. Maybe
>>> that's not a problem but if so, we really should change the default
>>> cpu model to be much more aggressive about exposing host features.
>>>
>> It's a tradeoff, but yes. We also need more sanity checks and
>> management commands giving management tools to understand whether
>> emulating guest CPU X on host CPU Y is safe/possible, and which guests
>> this might break.
>>
>
> I'd like to see Avi weigh in as historically, he's been one of the
> strongest advocates of a default migration policy of maximum
> compatibility. Personally, I think cross vendor migration is extremely
> uncommon in production and I would strongly recommend against it.
>
> My own experience has been that hardware homogeneity is pretty common in
> deployments, particularly in the processor space. There's such a
> different between Nehalem and non-Nehalem class processors that you
> really can't run the same workloads across the two without seeing a
> significant impact.
>
> So I'd actually be in favor of changing the default cpu to something
> that exposes a lot more of the underlying processor features. I think
> increased performance would be better for most consumers vs. increased
> migration flexibility.
>
> Then again, I'm certainly bias based on being employed by a hardware
> vendor :-)
>
> Regards,
>
> Anthony Liguori
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, (continued)
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Gleb Natapov, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Gleb Natapov, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm,
Michael S. Tsirkin <=
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/14
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/15
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/15
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Michael S. Tsirkin, 2009/12/15
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Avi Kivity, 2009/12/20
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Avi Kivity, 2009/12/20
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/20
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Avi Kivity, 2009/12/20
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Anthony Liguori, 2009/12/20
- Re: [Qemu-devel] cpuid problem in upstream qemu with kvm, Avi Kivity, 2009/12/20