Re: [RFC PATCH] Emulate MOVBE

[Gleb Natapov <gleb@redhat.com>]

On Tue, Apr 16, 2013 at 01:47:46PM +0200, Paolo Bonzini wrote:
> Il 10/04/2013 12:08, Gleb Natapov ha scritto:
> >> > What is the opinion from the KVM folks on this? Shall we start to
> >> > emulate instructions the host does not provide? In this particular case
> >> > a relatively simple patch fixes a problem (starting Atom optimized
> >> > kernels on non-Atom machines).
> > We can add the emulation, but we should not start announcing the instruction
> > availability to a guest if host cpu does not have it by default. This
> > may trick a guest into thinking that movbe is the fastest way to do
> > something when it is not.
> > 
> 
> 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.

--
			Gleb.