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

[Alexander Graf]

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.

> , 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);


Alex