Re: [Qemu-devel] KVMs default CPU type (was: allow sysenter on 32bit guests running on vmx host)

[Andrea Arcangeli]

On Fri, Jun 26, 2009 at 02:42:17AM +0200, Andrea Arcangeli wrote:
> that purely asks for troubles I think. At the same time I doubt we
> want to deviate much from qemu code here, this seems messy enough
> already without big changes to maintain in this area, and I guess this
> explains why kvm is only flipping the vendor_id right now...

Basically it seems athlon and other cpu definitions are tuned for cpus
having vmx/svm and running in legacy mode, that always support sep in
legacy mode and always supports syscall in long mode, so while qemu
seem to be cheating and not really emulating real hardware, those are
good tradeoffs definitions for KVM and it explains why it's enough to
flip the vendor_id to match the host vendor_id to get compatibility
mode right on 64bit guests, but only as long as 6/3/3 is set (hence
the reason of the patch).

So in short, we can't make any further change in KVM in addition to
waiting the common denominator to move to 6/3/3. First qemu has to
decide if it goes ahead not emulating real 32bit athlon but by
providing feature flags definition of a svm/vmx cpu in legacy mode.