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

[Gleb Natapov]

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.
> 
> > , 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);
> 
Are ARM cpu features not enumerable and the only way to know what is
supported by a core is by its id? I do see feature registers in the
spec, so why wouldn't we want to be able to disable/enable features
individually?

--
                        Gleb.