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Re: [Qemu-devel] [0/3] pseries: RFC: Advertise host CPU capabilties to g
Re: [Qemu-devel] [0/3] pseries: RFC: Advertise host CPU capabilties to guest
Fri, 30 Sep 2011 10:20:14 +0200
Am 30.09.2011 um 09:50 schrieb David Gibson <address@hidden>:
> This series contains some patches which, when using KVM, gather
> information about the capabilities of the host CPU and advertise them
> to the guest system when using the pseries machine. Specifically it
> does this for whether the CPU supports VMX, VSX and/or DFP
> instructions, and for the CPUs supported page sizes.
> The VSX and DFP portions of this were posted earlier, and I've fixed
> the minor comments which people made. This leaves one objection from
> Alex Graf, that whether the features are advertised should also depend
> on the target CPU selected in qemu. A similar objection may apply to
> the pagesizes patch. I guess the idea is to "clamp" the advertised
> capabilities to those permitted by the selected target CPU, but I'm
> not entirely sure what the logic here should be.
> Frankly, particularly in the case of KVM Book3S-HV, I'm not terribly
> convinced that attempting to make the guest CPU appear different from
> the host CPU is terribly meaningful. These patches as they stand have
> the advantage that future, roughly compatible CPUs should Just Work
> with these capabilities advertised in the correct cases. Alex, can
> you advise what sort of logic you'd like here.
Yes, very simple. I want you to create a CPU type 'host', similar to how x86
does it. That should be the default CPU type for KVM with the pseries machine.
You can also add a check in-kernel that verifies if guest PVR == host PVR for
HV mode. That way you ensure that -cpu host is always used there. If you later
add compat modes, you can check them there, but still have -cpu xxx available
to tell all pieces of the kvm/qemu chain what to use.