Re: [Qemu-devel] [PATCH] Add VMX cpuid feature to qemu64

[Nadav Har'El]

On Tue, Jan 04, 2011, Alexander Graf wrote about "Re: [Qemu-devel] [PATCH] Add 
VMX cpuid feature to qemu64":
> 
> On 04.01.2011, at 16:06, Nadav Har'El wrote:
> 
> > This patch adds the "VMX" cpuid feature to the default "qemu64" CPU type.
> > If KVM doesn't support this feature (i.e., nested VMX is not in the code,
> > or not enabled) it will mask out this bit.
> 
> "qemu64" defines capabilities that qemu emulates. Qemu does not emulate VMX. 
> So it doesn't belong there. I thought we had a "kvm64" type for your use case?

I have to apologize for making such beginner's mistakes, because I am not
very familiar with the inner workings of qemu.

It seemed to me, but please correct me if I'm wrong, that when I use qemu-kvm,
the default CPU type is "qemu64", not "kvm64", despite my using of KVM.
In this is true, why shouldn't qemu64 indeed include all the bits that KVM
may (perhaps) support? I wanted qemu-kvm to support nested VMX whenever the
underlying KVM supports this, without needing to resort to special "-cpu ..."
options.

I'm not sure what you meant by Qemu emulating SVM as the reason why SVM (but
not VMX) appears in qemu64's capabilities. As far as I understand, when you
use KVM, qemu doesn't emulate any of the CPU capabilities - rather, any
capability that KVM cannot support (such as nested SVM on an Intel processor)
are removed from the CPUID capabilties list.

Now that I think of it, perhaps I'm writing to the wrong mailing list?
Is there a separate mailing list for the development of qemu-kvm, as
opposed to the "general" qemu? I actually came here after being told by
the KVM developers to write to the "qemu-devel" mailing list...

> > Note that other relevant CPU types, such as "core2duo" already correctly
> > include the VMX feature, and "qemu64" already includes the SVM feature
> 
> "core2duo" really shouldn't include VMX as qemu simply can't emulate it. Qemu 
> does emulate SVM, hence the bit in the "qemu64" type.

Well, as you can see in target-i386/cpuid.c, at least in the qemu version
I checked out from KVM's repository, the "core2duo" and "coreduo" cpu types
do list the CPUID_EXT_VMX - and those are the only ones that list this feature.
I don't understand why they should have this bit, and qemu64 not, or,
alternatively - if none of those had the CPUID_EXT_VMX bit, how am I supposed
to run qemu to allow KVM to do nested VMX? I can think of some tricks (the
easiest is to use "-cpu host"), but why should these tricks be necessary
when they aren't for nested SVM? And when real core2duo CPUs *do* have VMX,
so if we try to accurately emulate them, we do need to support (nested) VMX?

Thanks in advance, and again I apologize if I'm missing the obvious, or
writing to the wrong mailing list.

-- 
Nadav Har'El                        |      Tuesday, Jan  4 2011, 29 Tevet 5771
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