Re: [PATCH 2/2] Allow the SVM CPUID bit in a VM

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> On a sidenote I'm really not fond of the userspace CPUID capability
> restrictions in the first place. Shouldn't this all be determined in the
> kernel module? It doesn't make since IMHO to restrict NX usage from
> userspace. The kernel module knows best what it's capable of.

I think cpuid handling will be a quite interesting when it comes to
merging qemu and kvm.  Doing it completely in the kernel isn't going to
work.  There are several reasons why you might want to change the cpuid
presented to the guest.

First, you might want to mask out cpuid bits not supported by all
machines in your pool, for migration compatibility.

Second, you might want to present a specific CPU to the guest (like qemu
-cpu pentium).  At least as close as possible, of course there are some
restrictions.  I think even if you mask out the sse cpuid bit sse
instructions will actually work.  Also having a virtual intel cpu on amd
and visa versa probably isn't going to work unless we'll fall back to
the emulator for the missing instructions.

Third, there is the hypervisor cpuid range (0x40000000+).  That one
needs to be configurable by userspace as well.

cheers,
  Gerd