Re: [RFC PATCH] Emulate MOVBE

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.

Paolo