Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic_map

Yang Zhang
2016-07-12T03:27:15Z

On 2016/7/11 23:52, Radim Krčmář wrote:
> 2016-07-11 16:14+0200, Paolo Bonzini:
>> On 11/07/2016 15:48, Radim Krčmář wrote:
>>>>> I guess the easiest solution is to replace kvm_apic_id with a field in
>>>>> struct kvm_lapic, which is already shifted right by 24 in xAPIC mode.
>>>
>>> (I guess the fewest LOC is to look at vcpu->vcpu_id, which is equal to
>>> x2apic id. xapic id cannot be greater than 255 and all of those are
>>> covered by the initial value of max_id.)
>>
>> Yes, this would work too. Or even better perhaps, look at vcpu->vcpu_id
>> in kvm_apic_id?
>
> APIC ID is writeable in xAPIC mode, which would make the implementation
> weird without an extra variable. Always read-only APIC ID would be
> best, IMO.
>
>>>> Or we can just simply put the assignment of apic_base to the end.
>>>
>>> Yes, this would work, I'd also remove recalculates from
>>> kvm_apic_set_*apic_id() and add a compiler barrier with comment for good
>>> measure, even though set_virtual_x2apic_mode() serves as one.
>>
>> Why a compiler barrier?
>
> True, it should be a proper pair of smp_wmb() and smp_rmb() in
> recalculate ... and current kvm_apic_id() reads in a wrong order, so
> changing the apic_base alone update wouldn't get rid of this race.
>
>>> (What makes a bit wary is that it doesn't avoid the same problem if we
>>> changed KVM to reset apic id to xapic id first when disabling apic.)
>>
>> Yes, this is why I prefer it fixed once and for all in kvm_apic_id...
>
> Seems most reasonable. We'll need to be careful to have a correct value
> in the apic page, but there shouldn't be any races there.

Yes, it is more reasonable.

>
>>> Races in recalculation and APIC ID changes also lead to invalid physical
>>> maps, which haven't been taken care of properly ...
>>
>> Hmm, true, but can be fixed separately. Probably the mutex should be
>> renamed so that it can be taken outside recalculate_apic_map...
>
> Good point, it'll make reasoning easier and shouldn't introduce any
> extra scalability issues.

If we can ensure all the updates to LDR,DFR,ID and apic mode are in
correct sequence and followed with apic map recalculation, it should be
enough. It's guest's responsibility to ensure the apic updating must
happen in right time(means no interrupt is in flying), otherwise the
interrupt may deliver to wrong VCPU.

--
best regards
yang