Re: [RFC PATCH] Emulate MOVBE

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Atom is not defined by QEMU; even if it was, it is unlikely to be
specified for a KVM guest since Atom doesn't support hardware
virtualization itself.

The next AMD processor that has MOVBE will probably have at least
another feature that is not in Opteron_G5, thus it will be the same as
Haswell.

Paolo