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Re: [libvirt] [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID



On Tue, Sep 24, 2013 at 01:04:14PM +0300, Gleb Natapov wrote:
> On Tue, Sep 24, 2013 at 11:57:00AM +0200, Borislav Petkov wrote:
> > On Mon, September 23, 2013 6:28 pm, Eduardo Habkost wrote:
> > > On Sun, Sep 22, 2013 at 04:44:50PM +0200, Borislav Petkov wrote:
> > >> From: Borislav Petkov <bp suse de>
> > >>
> > >> Add a kvm ioctl which states which system functionality kvm emulates.
> > >> The format used is that of CPUID and we return the corresponding CPUID
> > >> bits set for which we do emulate functionality.
> > >
> > > Let me check if I understood the purpose of the new ioctl correctly: the
> > > only reason for GET_EMULATED_CPUID to exist is to allow userspace to
> > > differentiate features that are native or that are emulated efficiently
> > > (GET_SUPPORTED_CPUID) and features that are emulated not very
> > > efficiently (GET_EMULATED_CPUID)?
> > 
> > Not only that - emulated features are not reported in CPUID so they
> > can be enabled only when specifically and explicitly requested, i.e.
> > "+movbe". Basically, you want to emulate that feature for the guest but
> > only for this specific guest - the others shouldn't see it.

Then we may have a problem: some CPU models already have "movbe"
included (e.g. Haswell), and patch 6/6 will make "-cpu Haswell" get
movbe enabled even if it is being emulated.

So if we really want to avoid enabling emulated features by mistake, we
may need a new CPU flag in addition to "enforce" to tell QEMU that it is
OK to enable emulated features (maybe "-cpu ...,emulate"?).

> > 
> > > If that's the case, how do we decide how efficient emulation should be,
> > > to deserve inclusion in GET_SUPPORTED_CPUID? I am guessing that the
> > > criterion will be: if enabling it doesn't risk making performance worse,
> > > it can get in GET_SUPPORTED_CPUID.
> > 
> > Well, in the MOVBE case, supported means, the host can execute this
> > instruction natively. Now, you guys say you can emulate x2apic very
> > efficiently and I'm guessing emulating x2apic doesn't bring any
> > emulation overhead, thus SUPPORTED_CPUID.
> x2apic emulation has nothing to do with x2apic in a host. It is emulated
> same way no matter if host has it or not. x2apic is not really cpu
> feature, but apic one and apic is fully emulated by KVM anyway.

But my question still stands: suppose we had x2apic emulation
implemented but for some reason it was painfully slow, we wouldn't want
to enable it by mistake. In this case, it would end up on EMULATED_CPUID
and not on SUPPORTED_CPUID, right?

> 
> > 
> > But for single instructions or group of instructions, the distinction
> > should be very clear.
> > 
> > At least this is how I see it but Gleb probably can comment too.
> > 
> That's how I see it two. Basically you want to use movbe emulation (as
> opposite of virtualization) only if you have binary kernel that compiled
> for CPU with movbe (Borislav's use case), or you want to migrate
> temporarily from movbe enabled host to non movbe host because downtime
> is not an option. We should avoid enabling it "by mistake".

"we should avoid enabling it 'by mistake'" sounds like a good criterion
for including something on GET_EMULATED_CPUID instead of
GET_SUPPORTED_CPUID.

In that case, I believe QEMU should use GET_EMULATED_CPUID only if
explicitly requested in the configuration/command-line (that's not what
patch 6/6 does).

-- 
Eduardo


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