Re: [Qemu-devel] [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID

[Borislav Petkov <bp@alien8.de>]

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
> EMULATED_CPUID and not on SUPPORTED_CPUID, right?

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

Thanks.

-- 
Regards/Gruss,
    Boris.

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