Re: [Qemu-devel] [PATCH] don't expose lm bit if kernel is not 64-bit capable.

[Anthony Liguori <aliguori@us.ibm.com>]

Glauber Costa wrote:
> On Tue, Feb 3, 2009 at 7:14 PM, Anthony Liguori <aliguori@us.ibm.com> wrote:
>   
>>
>> All I said was that if we used KVM_GET_SUPPORTED_CPUID, we had to do
>> something about save/restore since it can mask arbitrary things.
>>
>>     
> Then I misread you.
>
> Who's the subject of your sentence, that can mask arbitrary things?
> kvm.ko in its ioctl,
> or save/restore?
>   

Poor wording on my part.  Your previous patch called 
KVM_GET_SUPPORTED_CPUID which would return the host CPUID bits that KVM 
supports.  QEMU would then use that set to mask out features for the guest.

But the set of bits KVM returns differs on differing platforms.  This 
means that you're now enabling features that were not previously 
enabled.  This isn't a problem, but I'd like to see live migration 
handle this gracefully either by saving off the CPUID bits for the guest 
and failing appropriately when trying to enable CPUID bits on a platform 
that doesn't support it, or an explicit definition of CPUID capabilities.

I don't necessarily have a problem with enabling the most features by 
default, I just want migration to fail gracefully.

Regards,

Anthony Liguori