Re: [RFC PATCH] Emulate MOVBE

[Gleb Natapov <gleb@redhat.com>]

On Wed, Apr 17, 2013 at 12:42:30PM +0200, Paolo Bonzini wrote:
> Il 16/04/2013 19:28, Gleb Natapov ha scritto:
> > > This does highlight a weakness in CPU_GET_SUPPORTED_CPUID, but I think
> > > this is not a problem in practice.
> > > 
> > > With a management layer such as oVirt it's not a problem.  For example,
> > > oVirt has its own library of processors.  It doesn't care if KVM enables
> > > movbe.  If you tell it your datacenter is a mix of Haswells and Sandy
> > > Bridges it will pick the CPUID bits that are common to all.
> > > 
> > > However, even without a suitable management layer it is also not really
> > > a problem.
> > > 
> > > The only processors that support MOVBE are Atom and Haswell.  Haswell
> > > adds a whole lot of extra CPUID features, hence "-cpu Haswell,enforce"
> > > will fail with or without movbe emulation.  "-cpu Haswell" will disable
> > > all Haswell new features except movbe will remain slow; that's fine, I
> > > think, anyway it's not what you'ld do except to play with CPU models.
> >
> > No that's not fine. KVM should not trick userspace (QEMU is just one of
> > them) into nonoptimal configuration. And you forgot about -cpu host in your
> > analysis.
> 
> "-cpu host" enables bits that are not in the host, but that's a QEMU
> bug.  If the host lacks x2apic, for example, "-cpu host" should not
> enable it even if it is there in KVM_GET_SUPPORTED_CPUID.
> 
Why? We always said that if performance is the only thing user cares
about he should use "-cpu host". Not exposing emulated x2apic to a guest
will not provide best configuration possible. If user uses -cpu guest_cpu_model
and the guest_cpu_model is the same as host cpu model and host cpu
model does not have x2apic then guest should not see x2apic either, but
this works correctly today already.

> Now, KVM_GET_SUPPORTED_CPUID is a pretty bad interface for implementing
> a sane "CPU as similar to the host's as possible" policy.  That's
> another story and I agree.  It's a very sane policy for simple
> userspaces like lkvm.
> 
Thats because -cpu host is a misnomer. It really should be -cpu best
since its task is too create most efficient configuration given host
cpu. So you can wave -cpu host problem by saying that it should filter
non host cpuid bits from KVM_GET_SUPPORTED_CPUID, but then you will want
to implement -cpu best and you will encounter the same problem, but
without any excuse now :)

--
			Gleb.