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SubjectRe: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for HPT resizing
On Fri, Dec 16, 2016 at 10:25:55AM +0100, Thomas Huth wrote:
> On 15.12.2016 06:53, David Gibson wrote:
> > This adds a new powerpc-specific KVM_CAP_SPAPR_RESIZE_HPT capability to
> > advertise whether KVM is capable of handling the PAPR extensions for
> > resizing the hashed page table during guest runtime.
> >
> > At present, HPT resizing is possible with KVM PR without kernel
> > modification, since the HPT is managed within qemu. It's not possible yet
> > with KVM HV, because the HPT is managed by KVM. At present, qemu has to
> > use other capabilities which (by accident) reveal whether PR or HV is in
> > use to know if it can advertise HPT resizing capability to the guest.
> >
> > To avoid ambiguity with existing kernels, the encoding is a bit odd.
> > 0 means "unknown" since that's what previous kernels will return
> > 1 means "HPT resize possible if available if and only if the HPT is allocated in
> > userspace, rather than in the kernel". Userspace can check
> > KVM_CAP_PPC_ALLOC_HTAB to determine if that's the case. In practice
> > this will give the same results as userspace's fallback check.
> > 2 will mean "HPT resize available and implemented via ioctl()s
> > KVM_PPC_RESIZE_HPT_PREPARE and KVM_PPC_RESIZE_HPT_COMMIT"
>
> This encoding IMHO clearly needs some proper documentation in
> Documentation/virtual/kvm/api.txt ... and maybe also some dedicated
> #defines in an uapi header file.

Ah, yeah. Actually I'm talking to paulus again to see if we can come
up with a way to encode the necessary facts without something as weird
as this one.

--
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
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