On Sun, Feb 24, 2013 at 01:55:07PM +0000, Zhang, Yang Z wrote:
> > contexts, but only two use locks.
> See following logic, I think the problem you mentioned will not happened with current logic.
>
> get lock
> if test_pir (this will ensure there is no in flight IPI for same interrupt. Since we are taking the lock now, no IPI will be sent before we release the lock and no pir->irr is performed by hardware for same interrupt.)
> return coalesced
> if test_irr
> return coalesced
> set_pir
> injected=true
> if t_a_s(on) && in guest mode
> send ipi
> unlock
>
>
> >>> I'd rather think about proper way to do lockless injection before
> >>> committing anything. The case where we care about correct injection
> >>> status is rare, but we always care about scalability and since we
> >>> violate the spec by reading vapic page while vcpu is in non-root
> >>> operation anyway the result may be incorrect with or without the lock.
> >>> Our use can was not in HW designers mind when they designed this thing
> >>> obviously :(
> >>
> >> Zhang, can you comment on whether reading vapic page with CPU in
> >> VMX-non root accessing it is safe?
> See 24.11.4
> In addition to data in the VMCS region itself, VMX non-root operation can be controlled by data structures that are
> referenced by pointers in a VMCS (for example, the I/O bitmaps). While the pointers to these data structures are
> parts of the VMCS, the data structures themselves are not. They are not accessible using VMREAD and VMWRITE
> but by ordinary memory writes.
> Software should ensure that each such data structure is modified only when no logical processor with a current
> VMCS that references it is in VMX non-root operation. Doing otherwise may lead to unpredictable behavior.
>
> This means the initial design of KVM is wrong. It should not to modify vIRR directly.
>
> The good thing is that reading is ok.
OK.
> >> Gleb, yes, a comment mentioning the race (instead of the spinlock) and
> >> explanation why its believed to be benign (given how the injection
> >> return value is interpreted) could also work. Its ugly, though... murphy
> >> is around.
> > The race above is not benign. It will report interrupt as coalesced
> > while in reality it is injected. This may cause to many interrupt to be
> > injected. If this happens rare enough ntp may be able to fix time drift
> > resulted from this.
> Please check the above logic. Does it have same problem? If yes, please point out.
1) set_pir must be test_and_set_bit() (so the lock at vcpu entry path can
be dropped).
2) if t_a_s(on) && in guest mode
send ipi
should be inverted:
if guest mode && t_a_s(on)
send ipi
So you avoid setting ON bit if guest is outside of guest mode.
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