Re: [PATCH v7 3/3] x86, apicv: add virtual x2apic support

[Gleb Natapov <gleb@redhat.com>]

On Tue, Dec 25, 2012 at 07:25:15AM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2012-12-25:
> > On Tue, Dec 25, 2012 at 06:42:59AM +0000, Zhang, Yang Z wrote:
> >> Gleb Natapov wrote on 2012-12-25:
> >>> On Mon, Dec 24, 2012 at 11:53:37PM +0000, Zhang, Yang Z wrote:
> >>>> Gleb Natapov wrote on 2012-12-24:
> >>>>> On Mon, Dec 24, 2012 at 02:35:35AM +0000, Zhang, Yang Z wrote:
> >>>>>> Zhang, Yang Z wrote on 2012-12-24:
> >>>>>>> Gleb Natapov wrote on 2012-12-20:
> >>>>>>>> On Mon, Dec 17, 2012 at 01:30:50PM +0800, Yang Zhang wrote:
> >>>>>>>>> basically to benefit from apicv, we need clear MSR bitmap for
> >>>>>>>>> corresponding x2apic MSRs:
> >>>>>>>>>     0x800 - 0x8ff: no read intercept for apicv register virtualization
> >>>>>>>>>     TPR,EOI,SELF-IPI: no write intercept for virtual interrupt delivery
> >>>>>>>> We do not set "Virtualize x2APIC mode" bit in secondary execution
> >>>>>>>> control. If I read the spec correctly without that those MSR read/writes
> >>>>>>>> will go straight to physical local APIC.
> >>>>>>> Right. Now it cannot get benefit, but we may enable it in future and
> >>>>>>> then we can benefit from it.
> >>>>> Without enabling it you cannot disable MSR intercept for x2apic MSRs.
> >>>>> 
> >>>>>> how about to add the following check:
> >>>>>> if (apicv_enabled && virtual_x2apic_enabled)
> >>>>>> 	clear_msr();
> >>>>>> 
> >>>>> I do not understand what do you mean here.
> >>>> In this patch, it will clear MSR bitmap(0x800 -0x8ff) when apicv enabled. As
> > you
> >>> said, since kvm doesn't set "virtualize x2apic mode", APIC register
> >>> virtualization never take effect. So we need to clear MSR bitmap only
> >>> when apicv enabled and virtualize x2apic mode set.
> >>>> 
> >>> But currently it is never set.
> >> So you think the third patch is not necessary currently? Unless we
> >> enabled "virtualize x2apic mode".
> >> 
> > Without third patch vid will not work properly if a guest is in x2apic
> > mode. Actually second and third patches need to be reordered to not have
> > a windows where x2apic is broken. The problem is that this patch itself
> > is buggy since it does not set "virtualize x2apic mode" flag. It should
> > set the flag if vid is enabled and if the flag cannot be set vid should
> > be forced off.
> In what conditions this flag cannot be set? I think the only case is that KVM doesn't expose the x2apic capability to guest, if this is true, the guest will never use x2apic and we still can use vid.
> 
We can indeed set "virtualize x2apic mode" unconditionally since it does
not take any effect if x2apic MSRs are intercepted.

--
			Gleb.