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Re: [libvirt] [PATCH 1/1][RESEND] ppc64 cpu features

On 2013年04月22日 17:10, Jiri Denemark wrote:
On Mon, Apr 22, 2013 at 10:39:46 +0800, Li Zhang wrote:
On 2013年04月19日 22:04, Jiri Denemark wrote:
On Thu, Mar 14, 2013 at 14:54:21 +0800, Li Zhang wrote:
From: Li Zhang <zhlcindy linux vnet ibm com>

This patch adds a CPU feature "powernv" identifying IBM Power
processor that supports native hypervisor e.g. KVM. This can
be used by virtualization management software to determine
the CPU capabilities. PVR doesn't indicate whether it a
host or a guest CPU. So, we use /proc/cpuinfo to get the
platform information (PowerNV) and use that to set the
"powernv" flag.

Signed-off-by: Dipankar Sarma <dipankar in ibm com>
Signed-off-by: Li Zhang <zhlcindy linux vnet ibm com>
   src/cpu/cpu_map.xml    |    9 ++
   src/cpu/cpu_powerpc.c  |  349 ++++++++++++++++++++++++++++++++++++++----------
   src/cpu/cpu_ppc_data.h |    4 +
   src/util/sysinfo.c     |    2 +-
   4 files changed, 294 insertions(+), 70 deletions(-)
Looks like this patch was not even rebased since it was written back in
time 1.0.1 was released. Anyway, I realized I did not push my changes to
powerpc driver so I did that. And I also started rewriting this patch on
top of it since this patch is rather huge and seems to mix lots of
things. Also PowerPC CPUs seem to be quite different from x86 CPUs so
enhancing powerpc driver by copy&pasting code from x86 driver is not the
best way :-) It just makes powerpc driver unnecessarily complicated.
We hope it matches x86 driver, so most code are from x86.
There is no CPUID on powerpc, but we also need feature such as "powernv".
And CPUID is what makes x86 driver so complicated. That's why I think
it makes more sense to implement the right functionality from scratch
rather than copy&pasting x86 driver :-)

OK, you are right.
So what is this "powernv" feature used for? It won't show up in
capabilities XML as it is included in all powerpc CPU models. That also
This feature means that powerpc can support KVM.
Other platform, for example, pseries doesn't support KVM.
So we need this feature.

Currently, we haven't introduced this feature in capabilities XML yet.

means, users don't need to explicitly enable it when configuring guest
CPU. Thus it could only make sense to allow users to disable this
feature for a given guest. However, I don't see "powernv" string
anywhere in QEMU source code and thus it cannot really be used in any
way in guest CPU definition.
This feature is from sysinfo on host, not from QEMU.
QEMU haven't included this feature yet.
I am not sure whether it is necessary to introduce this to QEMU.
I think I understand what powernv feature is but I don't get what it is
(planned to be) used for in libvirt. And I'd like to understand that :-)
The CPU modeling stuff in libvirt is used for *guest* CPU configuration,
i.e., users can configure what CPU model and features the guest should
see. So unless you want it to enable/disable something like nested virt,
I don't see a lot of sense in introducing such feature. (I may be wrong
of course, that's why I'm asking.) Capabilities XML provides some
details about host CPU but that's mostly to give an idea about what
guest CPU configuration the host could be able to provide.

I see. This feature is read-only.
Nested virt isn't considered yet. Actually, I haven't understood this completely.
I think PowerPC doesn't support nest virt.
I just thought that migration is only allowed with 'powernv'.

Currently, only powernv platform can work with KVM.
In the future, there may be more features of CPU.

If you want to add powernv feature just because you need to distinguish
if the host supports KVM or not, there's much better way... In guest
section of capabilities XML, KVM support is indicated by <domain
type='kvm'> element. And that's what existing apps already use to detect
KVM presence/absence.

As my understanding, there is still some difference from 'kvm' capability.
'powernv' is only considered as one CPU feature of PPC64.
If other PPC platforms support KVM in the future, this feature can be used to
identify whether migration can be executed . :)



Li Zhang
IBM China Linux Technology Centre

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