Re: [Qemu-devel] [0/4] pseries: Support and improvements for KVM Book3S-HV support (v2)

[David Gibson <david@gibson.dropbear.id.au>]

On Fri, Oct 07, 2011 at 08:57:49AM +0200, Alexander Graf wrote:
> 
> On 30.09.2011, at 09:39, David Gibson wrote:
> 
> > Alex Graf has added support for KVM acceleration of the pseries
> > machine, using his Book3S-PR KVM variant, which runs the guest in
> > userspace, emulating supervisor operations.  Recent kernels now have
> > the Book3S-HV KVM variant which uses the hardware hypervisor features
> > of recent POWER CPUs.  Alex's changes to qemu are enough to get qemu
> > working roughly with Book3S-HV, but taking full advantage of this mode
> > needs more work.  This patch series makes a start on better exploiting
> > Book3S-HV.
> > 
> > Even with these patches, qemu won't quite be able to run on a current
> > Book3S-HV KVM kernel.  That's because current Book3S-HV requires guest
> > memory to be backed by hugepages, but qemu refuses to use hugepages
> > for guest memory unless KVM advertises CAP_SYNC_MMU, which Book3S-HV
> > does not currently do.  We're working on improvements to the KVM code
> > which will implement CAP_SYNC_MMU and allow smallpage backing of
> > guests, but they're not there yet.  So, in order to test Book3S-HV for
> > now you need to either:
> > 
> > * Hack the host kernel to lie and advertise CAP_SYNC_MMU even though
> >   it doesn't really implement it.
> > 
> > or
> > 
> > * Hack qemu so it does not check for CAP_SYNC_MMU when the -mem-path
> >   option is used.
> > 
> > Bot approaches are ugly and unsafe, but it seems we can generally get
> > away with it in practice.  Obviously this is only an interim hack
> > until the proper CAP_SYNC_MMU support is ready.
> 
> I would prefer the latter. We could even #ifdef it for TARGET_PPC.

Well, I don't see either approach as being remotely mergable.  So it's
really up to each individual person playing with it which hack is
easier for them to apply temporarily while waiting for the proper
solution to come along.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson