Re: [Qemu-devel] [RFC 0/2] GET_EMULATED_CPUID support with "allow-emulation" option

[Eduardo Habkost]

On Fri, Jun 06, 2014 at 03:21:04AM +0200, Borislav Petkov wrote:
> On Fri, Jun 06, 2014 at 12:24:26AM +0200, Alexander Graf wrote:
> > But can we drop the EMULATED name somehow? Can we rename [1] the ioctl
> > to say GET_UNSUPPORTED_CPUID or something along those lines? The name
> > is just a really really bad pick.
> 
> What do you mean, a "bad pick" :-P? I added extra care in naming that
> functionality what it is - bitfield in CPUID format of *emulated*
> features. Unsupported is wrong too - we do support them if we enable
> them explicitly. :-)
> 
> How about GET_NOT_REALLY_FAST_BUT_STILL_NOT_FAST_ENOUGH_AS_IN_HW_FAST_CPUID?

IMO, "emulated" on the kernel interface is good, because it describe
what it is. Deciding which emulated features are "experimental" or "good
enough to be enabled implicitly even if emulated" is policy for
userspace to decide.

"allow-experimental" is being mapped to GET_EMULATED_CPUID initially
only because _by default_ the GET_EMULATED_CPUID-only features won't be
enabled implicitly unless forced. But if one day we decide that
emulation is good enough for a specific feature, we can make
kvm_arch_get_supported_cpuid() return it even if it is present only on
the GET_EMULATED_CPUID bitmap. Even if that decision depends on a
specific implementation of that feature, the kernel can report that
using KVM capabilities (to be checked by kvm_arch_get_supported_cpuid(),
like we already do for tsc-deadline).

-- 
Eduardo