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Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support
Date: Wed, 21 May 2014 15:22:35 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 

On 21.05.14 14:56, Michael Mueller wrote:

On Tue, 20 May 2014 12:10:23 +0200
Alexander Graf <address@hidden> wrote:


On 20.05.14 12:02, Michael Mueller wrote:

On Mon, 19 May 2014 22:14:00 +0200
Alexander Graf <address@hidden> wrote:


On 19.05.14 19:03, Michael Mueller wrote:

On Mon, 19 May 2014 16:49:28 +0200
Alexander Graf <address@hidden> wrote:


[...]


What user and thus also user space wants depends on other factors:

1. reliability
2. performance
3. availability

It's not features, that's what programmers want.

That's why I have designed the model and migration capability around the 
hardware
and not around the software features and don't allow them to be enabled 
currently
together.

A software feature is a nice add on that is helpful for evaluation or 
development
purpose. There is few space for it on productions systems.

One option that I currently see to make software implemented facility migration
capable is to calculate some kind of hash value derived from the full set of
active software facilities. That value can be compared with pre-calculated
values also stored in the supported model table of qemu. This value could be
seen like a virtual model extension that has to match like the model name.

But I have said it elsewhere already, a soft facility should be an exception and
not the rule.


So all we need is a list of "features the guest sees available" which is
the same as "features user space wants the guest to see" which then gets
masked through "features the host can do in hardware".

For emulation we can just check on the global feature availability on
whether we should emulate them or not.


Also, if user space wants to make sure that its feature list is actually
workable on the host kernel, it needs to set and get the features again
and then compare that with the ones it set? That's different from x86's
cpuid implementation but probably workable.

User space will probe what facilities are available and match them with the 
predefined
cpu model set. Only those models which use a partial or full subset of the 
hard/host
facility list are selectable.

Why?

If a host does not offer the features required for a model it is not able to
run efficiently.


Please take a look at how x86 does cpuid masking :).

In fact, I'm not 100% convinced that it's a good idea to link cpuid /
feature list exposure to the guest and actual feature implementation
inside the guest together. On POWER there is a patch set pending that
implements these two things separately - admittedly mostly because
hardware sucks and we can't change the PVR.

That is maybe the big difference with s390. The cpuid in the S390 case is not
directly comparable with the processor version register of POWER.

In the S390 world we have a well defined CPU model room spanned by the machine
type and its GA count. Thus we can define a bijective mapping between
(type, ga) <-> (cpuid, ibc, facility set). From type and ga we form the model
name which BTW is meaningful also for a human user.

Same thing as POWER.


By means of this name, a management interface (libvirt) will draw decisions if
migration to a remote hypervisor is a good idea or not. For that it just needs
to compare if the current model of the guest on the source hypervisor
("query-cpu-model"), is contained in the supported model list of the target
hypervisor ("query-cpu-definitions").

I don't think this works, since QEMU should always return all the cpu
definitions it's aware of on query-cpu-definitions, not just the ones
that it thinks may be compatible with the host at a random point in time.

It does not return model names that it thinks they are compatible at some point
in time. In s390 mode, it returns all definitions (CPU models) that a given host
system is capable to run. Together with the CPU model run by the guest, some 
upper
management interface knows if the hypervisor supports the required CPU model and
uses a guest definition with the same CPU model on the target hypervisor.

The information for that is taken from the model table which QEMU builds up 
during
startup time. This list limits the command line selectable CPU models as well.

This makes s390 derive from the way x86 handles things. NAK.

One second, that goes a little fast here :-). x86 returns a list they support 
which happens
to be the full list they define and s390 does logically the same because we 
know that
certain models are not supported due to probing. BTW that happens only if you 
run Qemu on
back level hardware and that is perfectly correct.

It's not what other architectures do and I'd hate to see s390 deviate
just because.

Only these four architectures implement the query and they all differ a 
little...

target-arm/helper.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error 
**errp)
target-i386/cpu.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error 
**errp)
target-ppc/translate_init.c:CpuDefinitionInfoList 
*arch_query_cpu_definitions(Error **errp)
target-s390x/cpu.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error 
**errp)

arm walks through a list of all ARM CPU types
list = object_class_get_list(TYPE_ARM_CPU, false);
and returns the CpuDefinitionInfoList derived from that one to one

i386 loops over the static builtin_x86_defs[] array to retrieve the model names,
they don't even use the CPU class model as source

ppc walks through a list of all POWER CPU types
list = object_class_get_list(TYPE_POWERPC_CPU, false);
and then extends the produced list by all defined aliases

and s390x finally also walks through the defined S390 CPU types
list = object_class_get_list(TYPE_S390_CPU, false);
but drops those which are not usable (!is_active)
Just consider them as not defined. I actually would undefine
them if I knew how.

Also the commands comment says "list of supported virtual CPU definitions" and 
the s390
list contains all supported models, that's no contradiction.

So IMHO we can either

    a) change the definition of query_cpu_definitions to only return CPUs
that are executable with KVM on a given machine (probably a bad idea) or

no that is context dependent, yes in KVM case, no in TCG case. Actually for the 
TCG case one
would also report only those which can be emulated and not all.
Yup, we're already in a messy position there today with -cpu host which we only expose if KVM is available. 




    b) return not only the CPU type, but also a hint whether it's
available with KVM or

That is also changing the semantics


    c) add a parameter to query_cpu_definitions to say "only return KVM
runnable CPUs" or

That optional parameter is not used by libvirt


Well, we're trying to fix a generic bug here, so maybe it should be used ;).




    d) introduce a new query_kvm_cpu_definitions qmp command

The criterion is not KVM here, it is "supported by the current Qemu in 
conjuction with the given
host and kernel".
Only for KVM. For TCG it would be "supported by the current QEMU TCG engine on a given host". Maybe we'll implement transactional memory support in TCG one day but require host transactional memory support for it? 


If I compare the command with it's use on the Qemu Page
(http://wiki.qemu.org/Features/CPUModels) it says: "Requirement: libvirt needs 
to know which CPU
models are available to be used with the "-cpu" option. Current solution: 
libvirt uses QMP
query-cpu-definitions command."


##
# @query-cpu-definitions:
#
# Return a list of supported virtual CPU definitions
#
# Returns: a list of CpuDefInfo


The migration compatibility test is pretty much ARCH dependent. I looked into 
the
libvirt implementation and as one can see every architecture has its own 
implementation
there (libvirt/src/cpu/cpu_<arch>.c).

So here's my question again. How does x86 evaluate whether a target
machine is compatible with a source machine?

Will again look into that during the afternoon...

Yes, please. Someone else must have solved this before :).

Well, in my eyes it was newer solved for x86! The issue is that libvirt has its 
one x86 model and
feature code, independent from Qemu's model and feature implementation. You 
might know
libvirt's cpu_map.xml file where in the meantime also POWER started to add a 
hand-full of model
names. That's their source, they even use asm instructions to identify the 
local CPU and to
derive a model list they (libvirt) think to support. x86 requires alone more 
the 2500 lines of
code to implement its from qemu deviating model space. There is no test like 
the one that I
suggest that guarantees a priori a target hypervisor is capable to run a 
specific CPU model until
it fails.
Ok, maybe it's about time to start moving that code over into QEMU then. Maybe not. But this is not something you and me can just decide for s390 alone - I want this solved for everyone. 



x86 basically calculates a cpu model to be used from it's domain xml cpu 
statements, the cpuid
that it retrieves by itself and the feature definitions in the cpu_map.xml. 
During migartion
the domain xml is just copied without any change to the model. And eventually 
qemu startup might
fail.

That cpu_map.xml configuration is obsolete for other architectures like s390 
and also for arm.

The usable cpu model name space is retrieved by "virsh cpu-models s390x" which 
in our case is
taken from libvirt's qemu capabilities cache which gets filled by 
"query-cpu-definitions" during
libvirtd's startup time.

In addition we add a test at some point in libvirt's qemuDomainMigrateBegin() 
phase where we
verify the remote hypervisor is suitable to run the current domain's CPU model 
and to prepare
the destination domain XML where the CPU model might differ from the local.

Assume the local domain XML specifies host as current model, then the test will 
retrieve the
normalized model name by means of "query-cpu-model" and and use it to verify it 
with the
remote Qemu cpu model capabilities (sitting in the remote Qemu capabilities 
cache) and choose
the remote domain XML CPU model accordingly.
All of that sounds very reasonable. So now we only have to work on making that fit into the whole picture, not into a tiny s390 corner case. 


Alex
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