Re: [Qemu-devel] Will the ELI incorporated in theKVM?

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-09-19 16:43, Abel Gordon wrote:
> 
>> It's imperfect as you need to dedicate a core to pure guest-mode load
>> and cannot run userspace on that core (cannot walk through
>> userspace-based device models e.g.).
> 
> That's not correct.
> For the evaluation, we dedicated a core for each guest to maximize the
> performance but this
> is not a requirement. You can over-commit cores and share them across
> multiple VMs. In this case,
> you will be sharing the cycles among all the VMs and the performance per VM
> will be degraded.
> In addition, if you share a core, an interrupt for a guest may be raised
> while the corresponding
> VCPU is not running. Depending on the workload, most of the interrupts will
> be generated while
> the VCPU runs or not.
> 
>> and cannot run userspace on that core (cannot walk through
>> userspace-based device models e.g.).
> 
> That's not correct.
> We can run any thread (including user-space threads) on the same core we
> run the VMs (VCPU threads).
> In fact, we did that for the ELI evaluation: we shared a single core to run
> the VCPU thread and ALL the host threads (including qemu I/O thread).
> 
>> And it requires that magic bar to
>> map the shadow IDT into the guest (hmm, I think Hitachi avoided this).
> 
> Hitachi uses a different technique which seems to have the 2 disadvantages
> you previously mentioned while ELI doesn't have these disadvantages.
> 
>> It's invasive as it has to change Linux to maintain those isolated slave
>> CPUs. That is, of course, based on code that was published by Hitachi.
>> Yours may differ but will still have to solve the same problems.
> 
> ELI does not require isolated/slave CPUs.
> 

OK. Show patches.

Jan

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