Re: [Qemu-devel] [RFC 0/2] target-arm: Provide '-cpu host' when running KVM

[Alexander Graf]

On 14.08.2013, at 22:56, Christoffer Dall wrote:

> On Wed, Aug 14, 2013 at 10:47:08PM +0200, Alexander Graf wrote:
>> 
>> On 14.08.2013, at 22:33, Christoffer Dall wrote:
>> 
>>> On Wed, Aug 14, 2013 at 09:28:10PM +0200, Alexander Graf wrote:
>>>> 
>>>> On 14.08.2013, at 20:28, Christoffer Dall wrote:
>>>> 
>>>>> On Wed, Aug 14, 2013 at 08:21:54PM +0200, Alexander Graf wrote:
>>>>>> 
>>>>>> On 14.08.2013, at 20:18, Christoffer Dall wrote:
>>>>>> 
>>>>>>> On Wed, Aug 14, 2013 at 07:44:25PM +0200, Alexander Graf wrote:
>>>>>>>> 
>>>>>>>> On 14.08.2013, at 19:39, Christoffer Dall wrote:
>>>>>>>> 
>>>>>>>>> On Wed, Aug 14, 2013 at 07:31:59PM +0200, Alexander Graf wrote:
>>>>>>>>>> 
>>>>>>>>>> On 14.08.2013, at 19:26, Christoffer Dall wrote:
>>>>>>>>>> 
>>>>>>>>>>> On Wed, Aug 14, 2013 at 11:30:46AM +0200, Alexander Graf wrote:
>>>>>>>>>>>> 
>>>>>>>>>>>> On 14.08.2013, at 11:23, Peter Maydell wrote:
>>>>>>>>>>>> 
>>>>>>>>>>>>> On 14 August 2013 10:11, Alexander Graf <address@hidden> wrote:
>>>>>>>>>>>>>> You're right, the main difference is that KVM doesn't have any
>>>>>>>>>>>>>> idea what a "host" style CPU is. It only knows how to report to 
>>>>>>>>>>>>>> QEMU
>>>>>>>>>>>>>> what the current host CPU would be, so that anything from 
>>>>>>>>>>>>>> VCPU_INIT
>>>>>>>>>>>>>> onwards is 100% identical regardless of whether the user said
>>>>>>>>>>>>>> -cpu host or -cpu xxx.
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> I'm still puzzled on how this will work with BIG.little btw.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> The rough idea is that for BIG.little the kernel must trap the
>>>>>>>>>>>>> ID registers at least (so that the vcpu seems consistent to the
>>>>>>>>>>>>> guest whether it's running on the big or the little core). For
>>>>>>>>>>>>> "-cpu host" the guest would see whatever is the most low-overhead
>>>>>>>>>>>>> for the kernel to provide (ie assuming the big and little CPUs
>>>>>>>>>>>>> are roughly-similar you could make -cpu host provide something
>>>>>>>>>>>>> that looks to the guest like the big CPU and don't have to trap
>>>>>>>>>>>>> quite as much as you would for providing a vcpu that wasn't the
>>>>>>>>>>>>> same as either the big or little one).
>>>>>>>>>>>> 
>>>>>>>>>>>> So -cpu host in this case wouldn't actually expose the host CPU 
>>>>>>>>>>>> 1:1, but instead a cortex-a15 even when it's run on an a7 
>>>>>>>>>>>> BIG.little core. I see.
>>>>>>>>>>>> 
>>>>>>>>>>> Yes, from the discussion we've had the whole picture just becomes to
>>>>>>>>>>> blurry when you start presenting multiple different CPUs to the 
>>>>>>>>>>> guest
>>>>>>>>>>> and there's really no need to that I'm aware of.
>>>>>>>>>>> 
>>>>>>>>>>> In fact the -cpu host case fits quite nicely with this state of 
>>>>>>>>>>> mind;
>>>>>>>>>>> the kernel is free to decide based on the specific hardware and 
>>>>>>>>>>> config
>>>>>>>>>>> on which it's running how to handle VMs on BL.
>>>>>>>>>> 
>>>>>>>>>> So why not have a vm ioctl to fetch the "best match" vcpu type? I 
>>>>>>>>>> don't like the idea of adding any awareness of a "host" type to the 
>>>>>>>>>> normal vcpu creation process.
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>> That's actually what I suggested initially.  I'm not really a QEMU
>>>>>>>>> expert, but I think Peter already answered this question: he doesn't
>>>>>>>>> want to support hundreds of CPU models in QEMU just to be able to run
>>>>>>>>> KVM when it's not necessary.
>>>>>>>>> 
>>>>>>>>> If his argument holds in that you can support -cpu host without 
>>>>>>>>> having a
>>>>>>>>> model for that specific cpu in QEMU, then indeed it is a strong
>>>>>>>>> argument, and we have the problem with ARMv8 already, and this goes a
>>>>>>>>> long way to solve that. No?
>>>>>>>> 
>>>>>>>> That's up for QEMU to decide. With the "fetch and push" model we can 
>>>>>>>> support both flavors from user space. It also makes the kernel side 
>>>>>>>> more reproducible and obvious. There's simply no way to add hacks like 
>>>>>>>> "If I'm a -cpu host type do xxx" in KVM, because KVM never knows that 
>>>>>>>> it is running -cpu host.
>>>>>>>> 
>>>>>>> Do we have historical examples of this knowledge being abused inside the
>>>>>>> kernel for other archs?  If not, can we come up with a technical
>>>>>>> scenario where it may happen on ARM?
>>>>>> 
>>>>>> if (cpu == host_cpu) {
>>>>>> vgic_version = get_host_vgic_version();
>>>>>> }
>>>>>> 
>>>>>> would be an example :). 
>>>>> 
>>>>> Not really, this is driven from user space, but ok.
>>>>> 
>>>>>> Everything -cpu host does has to be reproducible without -cpu host, 
>>>>>> otherwise our compatibility layering is broken. So why not model the API 
>>>>>> like it from the beginning?
>>>>>> 
>>>>>>> 
>>>>>>> Also, not really sure if such code should be controlled through the user
>>>>>>> space API; ideally we would catch bad coding behavior in the kernel
>>>>>>> during code review.
>>>>>>> 
>>>>>>> The only reason I originally suggested the "fetch and push" model was
>>>>>>> that I thought user space would need to know the specific CPU model for
>>>>>>> things to work and possibly for things like debugging and migration, but
>>>>>>> since I have been almost convinced otherwise, I don't see any real
>>>>>>> technical arguments for not adding -cpu host support in the kernel side.
>>>>>>> 
>>>>>>> Note that this doesn't prevent us from adding an IOCTL later that gives
>>>>>>> you the host CPU type in KVM terminology if we find it useful.  But, I
>>>>>>> think the reduced headache with ARMv8 right now is a good argument to
>>>>>>> proceed with Peter's RFC and kernel support for same.
>>>>>> 
>>>>>> There's really almost no difference from the QEMU point of view if Peter 
>>>>>> choses to implement it the way he does today. He can ask the kernel for 
>>>>>> the vcpu target and pass that exact number back into the kernel.
>>>>>> 
>>>>>> 
>>>>> From the kernel point of view though we have to make some informed
>>>>> decision about which "best CPU target" value to return on any given new
>>>>> core
>>>> 
>>>> We have to make that decision internally anyways, because we have to 
>>>> choose some CPU target for the host one.
>>>> 
>>> 
>>> Are we sure that will always be the case?  That's how it's structured
>>> now, yes, but maybe we can do something more intelligent (which is what
>>> I meant with "generic handling" below).  It's a bit fuzzy for me to
>>> think about right now, but I just want to make sure we don't shoot
>>> ourselves in the foot with the choice of ABI.
>> 
>> Exactly that cleverness is what I'd prefer to avoid, as it breaks 
>> reproducibility with cross-chip environments.
>> 
>>> 
>>>>> , where TARGET_HOST may simply work through generic handling of id
>>>>> registers, traps etc. and provide better performance than say, "I don't
>>>>> really know this host CPU, so I'm just going to tell you A15 and trap
>>>>> everything"...
>>>> 
>>>> Yes.
>>>> 
>>>> target_vcpu_id = kvm_vm_ioctl(KVM_VM_GET_BEST_CPU_TARGET);
>>>> 
>>>> /* Old kernels only support A15 hosts */
>>>> if (target_vcpu_id < 0)
>>>> target_vcpu_id = VCPU_CORTEX_A15;
>>>> 
>>>> kvm_vcpu_ioctl(vcpu_fd, KVM_INIT_VCPU, target_vcpu_id);
>>>> 
>>> 
>>> I get this part, but imagine the kernel not knowing the target_cpu id,
>>> but just passing through whatever the hardware gives you to the guest.
>> 
>> Don't you have to handle core specific registers anyway?
>> 
>>> I'm not saying that's necessarily going to happen or that it would be a
>>> great thing, but do we want to prevent this from ever happening through
>>> our choice of ABI?
>> 
>> I think so, yes. Can you run Linux on a core that hasn't been enabled? Why 
>> should you be able to run KVM on a core that hasn't been enabled? I'm not 
>> talking about QEMU here - that one should be happy to be ignorant. But the 
>> kernel side needs to know about the core either way, no?
>> 
>> 
> I guess so, maybe.  So far we haven't seen a lot of cores, so we
> definietely want to know about the specific cores that we want to
> support.
> 
> But, if there really are going to be hundreds, or thousands, or hundreds
> of thousands of virtualization-enabled ARM cores (ok, maybe not) then

We're talking about cores here. My take is that we're rather looking at dozens 
at most.

> maybe we want to come up with a way to not add code in KVM for every new
> core we wish to support.  But then again, if ARM achieves true world
> domination and inflicts us with that many core types, we can always
> add a new ABI.

If the need really emerges we can always still add a TARGET_HOST target type 
that the enumeration function returns. But I don't think we'll ever hit that 
case.

> So at this point, I don't really care if we do it once way or another.

Great :).


Alex