[libvirt] [Qemu-devel] [PATCH 6/7] target-i386: add implementation of query-cpudefs
Anthony Liguori
aliguori at us.ibm.com
Fri Aug 10 17:09:44 UTC 2012
Eduardo Habkost <ehabkost at redhat.com> writes:
> On Fri, Aug 10, 2012 at 11:37:30AM -0500, Anthony Liguori wrote:
>> Eduardo Habkost <ehabkost at redhat.com> writes:
>> >> > - add machine-type-specific cpudef compatibility changes?
>> >>
>> >> I think we've discussed this in IRC. I don't think we need to worry
>> >> about this.
>> >
>> > I remember discussing a lot about the mechanism we will use to add the
>> > compatibility changes, but I don t know how the query API will look
>> > like, after we implement this mechanism.
>>
>> 0) User-defined CPU definitions go away
>> - We already made a big step in this direction
>>
>> 1) CPU becomes a DeviceState
>
> 1.1) CPU models become classes
>
>>
>> 2) Features are expressed as properties
>>
>> 3) Same global mechanism used for everything else is used for CPUs
>
> This is basically the compatibility mechanism we agreed upon, yes, but
> what about the probing mechanism to allow libvirt to know what will be
> the result of "-machine M -cpu C"[1] before actually starting a VM?
I think that the requirement of "before actually starting a VM" is
unreasonable.
Presumably migration compatibility checking would happen after launching
a guest so libvirt could surely delay querying the CPUID info until
after the guest has started.
There's a lot of logic involved in deciding what gets exposed to the
guest. We don't really fully know until we've created the VCPU. It's a
whole lot easier and saner to just create the VCPU.
Regards,
Anthony Liguori
>
> [1] By "result" I mean:
> - Whether that combination can be run properly on that host;
> - Which CPU features will be visible to the guest in case it runs.
> Both items depend on CPU model _and_ machine-type, that's why we need
> some probing mechanism that depends on the machine-type or use the
> machine-type as input.
>
>
>>
>> Regards,
>>
>> Anthony Liguori
>>
>> >> > Would the command report different results depending on -machine?
>> >>
>> >> No.
>> >
>> > The problem is:
>> >
>> > 1) We need to introduce fixes on a CPU model that changes the set of
>> > guest-visible features (add or remove a feature)[1];
>> > 2) The fix has to keep compatibility, so older machine-types will
>> > keep exposing the old set of gues-visible features;
>> > - That means different machine-types will have different CPU
>> > features being exposed.
>> > 3) libvirt needs to control/know which guest-visible CPU features are
>> > available to the guest (see above);
>> > 4) Because of (2), the querying system used by libvirt need to depend on
>> > the CPU model and machine-type.
>> >
>> >
>> > [1] Example:
>> > The SandyBridge model today has the "tsc-deadline" bit set, but
>> > QEMU-1.1 did not expose the tsc-deadline feature properly because of
>> > incorrect expectations about the GET_SUPPORTED_CPUID ioctl. This was
>> > fixed on qemu-1.2.
>> >
>> > That means "qemu-1.1 -machine pc-1.1 -cpu SandyBridge" does _not_
>> > expose tsc-deadline to the guest, and we need to make "qemu-1.2
>> > -machine pc-1.1 -cpu SandyBridge" _not_ expose it, too (otherwise
>> > migration from qemu-1.1 to qemu-1.2 will be broken).
>> >
>> >>
>> >> >
>> >> > Would the command return the latest cpudef without any machine-type
>> >> > hacks, and libvirt would have to query for the cpudef compatibility data
>> >> > for each machine-type and combine both pieces of information itself?
>> >>
>> >> I'm not sure what you mean by compatibility data.
>> >
>> > I mean any guest-visible compatibility bit that we will need to
>> > introduce on older machine-types, when making changes on CPU models (see
>> > the SandyBridge + tsc-deadline example above).
>> >
>> > I see two options:
>> > - Libvirt queries for a [f(machine_type, cpu_model) -> cpu_features]
>> > function, that will take into account the machine-type-specific
>> > compatibility bits.
>> > - Libvirt queries for a [f(cpu_model) -> cpu_features] function and a
>> > [f(machine_type) -> compatibility_changes] function, and combine both.
>> > - I don't like this approach, I am just including it as a possible
>> > alternative.
>> >
>> >>
>> >> Regards,
>> >>
>> >> Anthony Liguori
>> >>
>> >> >
>> >> > [1] Note that it doesn't have to be low-level leaf-by-leaf
>> >> > register-by-register CPUID bits (I prefer a more high-level
>> >> > interface, myself), but it has to at least say "feature FOO is
>> >> > enabled/disabled" for a set of features libvirt cares about.
>> >> >
>> >> > --
>> >> > Eduardo
>> >>
>> >
>> > --
>> > Eduardo
>>
>
> --
> Eduardo
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