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Re: [Qemu-devel] [0/3] pseries: RFC: Advertise host CPU capabilties to g

From: Alexander Graf
Subject: Re: [Qemu-devel] [0/3] pseries: RFC: Advertise host CPU capabilties to guest
Date: 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.


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