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

On Thu, Sep 26, 2013 at 04:20:59PM -0300, Eduardo Habkost wrote:
> Please point me to the code that does this, because I don't see it on
> patch 6/6.

@@ -1850,7 +1850,14 @@ static void filter_features_for_kvm(X86CPU *cpu)
         uint32_t requested_features = env->features[w];
+        uint32_t emul_features = kvm_arch_get_emulated_cpuid(s, wi->cpuid_eax,
+                                                                wi->cpuid_ecx,
+                                                                wi->cpuid_reg);
         env->features[w] &= host_feat;
+        env->features[w] |= (requested_features & emul_features);

Basically we give the requested_features a second chance here.

If we don't request an emulated feature, it won't get enabled.

> > If you start with "-cpu Haswell", MOVBE
> > will be already set in the host CPUID.
> > 
> > Or am I missing something?
> In the Haswell example, it is unlikely but possible in theory: you would
> need a CPU that supported all features from Haswell except movbe. But
> what will happen if you are using "-cpu n270,enforce" on a SandyBridge
> host?

That's an interesting question: AFAICT, it will fail because MOVBE is
not available on the host, right?

And if so, then this is correct behavior IMHO, or how exactly is the
"enforce" thing supposed to work? Enforce host CPUID?

> Also, we don't know anything about future CPUs or future features
> that will end up on EMULATED_CPUID. The current code doesn't have
> anything to differentiate features that were already included in the
> CPU definition and ones explicitly enabled in the command-line (and I
> would like to keep it that way).


> And just because a feature was explicitly enabled in the command-line,
> that doesn't mean the user believe it is acceptable to get it running
> in emulated mode. That's why I propose a new "emulate" flag, to allow
> features to be enabled in emulated mode.

And I think, saying "-cpu ...,+movbe" is an explicit statement enough to
say that yes, I am starting this guest and I want MOVBE emulation.

> Well, x2apic is emulated by KVM, and it is on SUPPORTED_CPUID. Ditto
> for tsc-deadline. Or are you talking specifically about instruction
> emulation?

Basically, I'm viewing this from a very practical standpoint - if I
build a kernel which requires MOVBE support but I cannot boot it in kvm
because it doesn't emulate MOVBE (TCG does now but it didn't before)
I'd like to be able to address that shortcoming by emulating that
instruction, if possible.

And the whole discussion grew out from the standpoint of being able to
emulate stuff so that you can do quick and dirty booting of kernels but
not show that emulation capability to the wide audience since it is slow
and it shouldn't be used and then migration has issues, etc, etc.

But hey, I don't really care all that much if I have to also say
-emulate in order to get my functionality.



Sent from a fat crate under my desk. Formatting is fine.

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