On Sun, Apr 21, 2013 at 12:46:51PM +0300, Gleb Natapov wrote:
> There may be userspaces that set ->function to 0xffffffff (just
> because they do not init the buffer before calling into the kernel)
> and this will break them.
You don't mean 0xffffffff here but rather something random which is not
properly initialized and by chance is a valid CPUID leaf. Then, if some
of the bits in the register variables e[abcd]x are also set, we return
with emulated bits set.
> > For each set bit, we check whether we emulate the respective feature
> > and if so, we leave it untouched before returning it to userspace.
> > Otherwise, we clear it before OR-ing in the host bits and the
> > good-emulated bits like x2apic.
> >
> > Yeah, semantics need to be handled carefully, but it has this
> > knock-on-door aspect where kvm says that it actually emulates a feature
> > only if asked, i.e. with the -cpu ...,+<feature> syntax.
> >
> > * new ioctl KVM_GET_EMULATED_CPUID
> >
> > Might be overkill and might be used only in a limited fashion since we
> > don't want to emulate *every* feature in kvm.
> >
> > Hmmm. I kinda like the first one more while the second one is cleaner.
> >
> The first one needs explicit userspace support to work correctly. This
> should be other way around: old userspace should do the right thing, but may
> not support new features, new userspace should be able to support new
> feature.
Crap, not even qemu is handing in cleared buffers with that g_malloc0()
thing, AFAICT.
> We may extend KVM_GET_SUPPORTED_CPUID instead of adding new one. There
> is a padding field in kvm_cpuid2 that we could have reused as flags,
That would've been lovely.
> but unfortunately current implementation does not error if the field is
> not zero, so if there is a userspace that does not zero the padding it may
> break. Another options is to reuse high bits of nent as flags (not very
> nice, but will work).
Not nice?! It is a very nasty hack - that's what it is. :-)
Frankly speaking, I'd rather prefer adding a new ioctl. Since old
userspace won't support the new features, then we just as well can
simply add the new ioctl.
In all the cases we need to touch userspace - be it to OR in the flags
into ->nent or to implement the new ioctl. So why not do it in a clean
manner from the get-go?
Hmmm.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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