Re: unconditional CPUID propagation?

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On Wed, Aug 03, 2011 at 11:41:42AM +0200, Andre Przywara wrote:
> Hi,
> 
> while looking through the code I found commit
> f79116867ec80ed5d1d10043a3fd9ac8afd182c1 (upstream QEMU: enable
> SMEP) which unconditionally propagates the bits from CPUID leaf 0x7
> to the guest. Though there is the KVM module in the line, this
> currently whitelists three feature bits.
> Doesn't that break migration? The result of the CPUID instruction
> the guess issues only depends on the host and the KVM module's
> policy, not on the CPU model QEMU uses. So I guess migrating from a
> newer CPU to an older one breaks despite a rather conservative CPU
> model has been chosen intentionally by the user.
> The same is probably true for the VIA CPUID leaf.
> 
> Is that considered OK now or is that a bug? Shall the new feature
> bits be made known to QEMU like the other ones on only enabled
> explicitly (+smep) or by -cpu host?
> I can make a patch for that if that is the right way to address this.

Or if the CPU type supports it, yes.

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