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SubjectRe: [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID
On Thu, Sep 26, 2013 at 11:19:15AM -0300, Eduardo Habkost wrote:
> 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.

Huh? HSW has MOVBE so we won't #UD on it and MOVBE will get executed in
hardware when executing the guest. IOW, we'll never get to the emulation
path of piggybacking on the #UD.

> 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"?).

EMULATED_CPUID are off by default and only if you request them
specifically, they get enabled. If you start with "-cpu Haswell", MOVBE
will be already set in the host CPUID.

Or am I missing something?

> 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

IMHO we want to enable emulation only when explicitly requested...
regardless of the emulation performance.



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

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