linux-kernel - Re: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for HPT resizing

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Date:   Mon, 19 Dec 2016 17:37:18 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Thomas Huth <thuth@...hat.com>
Cc:     paulus@...ba.org, michael@...erman.id.au, benh@...nel.crashing.org,
        sjitindarsingh@...il.com, lvivier@...hat.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for
 HPT resizing

On Fri, Dec 16, 2016 at 02:15:30PM +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"
> > 
> > For now we always return 1, but the intention is to return 2 once HPT
> > resize is implemented for KVM HV.
> > 
> > Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> > ---
> >  arch/powerpc/kvm/powerpc.c |  3 +++
> >  include/uapi/linux/kvm.h   | 10 ++++++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index efd1183..bb23923 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -605,6 +605,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  	case KVM_CAP_SPAPR_MULTITCE:
> >  		r = 1;
> >  		break;
> > +	case KVM_CAP_SPAPR_RESIZE_HPT:
> > +		r = 1; /* resize allowed only if HPT is outside kernel */
> > +		break;
> >  #endif
> >  	case KVM_CAP_PPC_HTM:
> >  		r = cpu_has_feature(CPU_FTR_TM_COMP) &&
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index cac48ed..904afe0 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -685,6 +685,12 @@ struct kvm_ppc_smmu_info {
> >  	struct kvm_ppc_one_seg_page_size sps[KVM_PPC_PAGE_SIZES_MAX_SZ];
> >  };
> >  
> > +/* for KVM_PPC_RESIZE_HPT_{PREPARE,COMMIT} */
> > +struct kvm_ppc_resize_hpt {
> > +	__u64 flags;
> > +	__u32 shift;
> > +};
> 
> I think you should also add a final "__u32 pad" to that struct to make
> sure that it is naturally aligned (like it is done with struct
> kvm_coalesced_mmio_zone already for example).

Seems reasonable; done.

-- 
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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