Re: CPUID emulation

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Stephane Eranian wrote:
> Hello,
>
> Looking at kvm-26, it seems that the CPUID values as seen by the guest OS
> are still hardcoded for i386/x86-64 at least.
>
> For performance counter virtualization, the guest needs to see the *actual*
> family/model information in order to correctly program the counters.
>
> It would be fairly simple to grab that information from /proc/cpuinfo
> at init time in qemu.
>
> However, I am wondering if there would be side effects of using the
> actual CPUID which could cause troubles to KVM or the guest.
>
>   

The main issue is migration (and save/restore).  If you migrate to a 
host with fewer capabilities, applications may fail when they use 
unavailable instructions that they successfully detected earlier.

I think the best solution is to default to the host's capabilities, but 
allow command line switches to override them.  This way a management 
application in a server farm can set a site-wide least common 
denominator, while a home user can enjoy the latest and greatest 
instructions on their machine.

Upstream qemu already has a -cpu (or similar) switch for non-x86; we can 
probably use that.

(there's another possible issue - some future features may require 
support from the hypervisor - that may conflict with defaulting to to 
the host feature set.  maybe kvm should mask out any unknown features)

-- 
error compiling committee.c: too many arguments to function


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