On 08/04/20 01:21, Thomas Gleixner wrote: > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > >> On 07/04/20 22:20, Thomas Gleixner wrote: >>>>> Havind said that, I thought disabling interrupts does not mask exceptions. >>>>> So page fault exception should have been delivered even with interrupts >>>>> disabled. Is that correct? May be there was no vm exit/entry during >>>>> those 10 seconds and that's why. >>> No. Async PF is not a real exception. It has interrupt semantics and it >>> can only be injected when the guest has interrupts enabled. It's bad >>> design. >> >> Page-ready async PF has interrupt semantics. >> >> Page-not-present async PF however does not have interrupt semantics, it >> has to be injected immediately or not at all (falling back to host page >> fault in the latter case). > > If interrupts are disabled in the guest then it is NOT injected and the > guest is suspended. So it HAS interrupt semantics. Conditional ones, > i.e. if interrupts are disabled, bail, if not then inject it. Interrupts can be delayed by TPR or STI/MOV SS interrupt window, async page faults cannot (again, not the page-ready kind). Page-not-present async page faults are almost a perfect match for the hardware use of #VE (and it might even be possible to let the processor deliver the exceptions). There are other advantages: - the only real problem with using #PF (with or without KVM_ASYNC_PF_SEND_ALWAYS) seems to be the NMI reentrancy issue, which would not be there for #VE. - #VE are combined the right way with other exceptions (the benign/contributory/pagefault stuff) - adjusting KVM and Linux to use #VE instead of #PF would be less than 100 lines of code. Paolo > But that does not make it an exception by any means. > > It never should have been hooked to #PF in the first place and it never > should have been named that way. The functionality is to opportunisticly > tell the guest to do some other stuff. > > So the proper name for this seperate interrupt vector would be: > > VECTOR_OMG_DOS - Opportunisticly Make Guest Do Other Stuff > > and the counter part > > VECTOR_STOP_DOS - Stop Doing Other Stuff > >> So page-not-present async PF definitely needs to be an exception, this >> is independent of whether it can be injected when IF=0. > > That wants to be a straight #PF. See my reply to Andy. > >> Hypervisors do not have any reserved exception vector, and must use >> vectors up to 31, which is why I believe #PF was used in the first place >> (though that predates my involvement in KVM by a few years). > > No. That was just bad taste or something worse. It has nothing to do > with exceptions, see above. Stop proliferating the confusion. > >> These days, #VE would be a much better exception to use instead (and >> it also has a defined mechanism to avoid reentrancy). > > #VE is not going to solve anything. > > The idea of OMG_DOS is to (opportunisticly) avoid that the guest (and > perhaps host) sit idle waiting for I/O until the fault has been > resolved. That makes sense as there might be enough other stuff to do > which does not depend on that particular page. If not then fine, the > guest will go idle. > > Thanks, > > tglx >
