Re: [Qemu-devel] Re: [PATCH 3/4] mask out forbidden cpuid features with kvm ioctl

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> Glauber Costa wrote:
>> KVM has a (so far unused) ioctl to inform userspace
>> about supported cpuid bits in the current host.
>>
>> The lack of this kind of checking can lead to bugs in which
>> a cpuid bit is exposed but not supported by the host kernel
>> (an example is fedora bug 
>> https://bugzilla.redhat.com/show_bug.cgi?id=481274)
>>
>> The current host_cpuid code is replaced by this general check.
>>
>> Signed-off-by: Glauber Costa <glommer@redhat.com>
>>   
>
> So what do we do about live migration?  

We need a way to completely specify cpuid from the command line.  The 
management utility would figure out what is supported (driven by both 
cpu capabilities and installed kvm capabilities) and fgure out a 
greatest common denominator, or something.

> Does KVM mask a set of bits unconditionally?  

The kernel doesn't touch cpuid (except nx due to an historical mistake).

> Before, we were masking unconditionally which should insulate 
> migration reasonable well.

That suits migration, but not other uses of qemu.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.