Subject: Re: [PATCH 01/11] powerpc/kvm: Reserve capabilities and ioctls for
 HPT resizing
To: David Gibson <david@gibson.dropbear.id.au>, paulus@samba.org
References: <20161215055404.29351-1-david@gibson.dropbear.id.au>
 <20161215055404.29351-2-david@gibson.dropbear.id.au>
Cc: michael@ellerman.id.au, benh@kernel.crashing.org,
        sjitindarsingh@gmail.com, lvivier@redhat.com,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
From: Thomas Huth <thuth@redhat.com>
Message-ID: <25bcced1-4903-2cf7-6a56-14c3ced5cd28@redhat.com>
Date: Fri, 16 Dec 2016 10:25:55 +0100
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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.

 Thomas