On Thu, Dec 07, 2023 at 06:09:46PM +0800, Li, Xin3 wrote: >> >> > Exiting-event identification can also have bit 13 set, indicating a >> >> > nested exception encountered and caused VM-exit. when reinjecting the >> >> > exception to guests, kvm needs to set the "nested" bit, right? I >> >> > suspect some changes to e.g., handle_exception_nmi() are needed. >> >> >> >> The current patch relies on kvm_multiple_exception() to do that. But TBH, I'm >> >> not sure it can recognize all nested cases. I probably should revisit it. >> > >> >So the conclusion is that kvm_multiple_exception() is smart enough, and >> >a VMM doesn't have to check bit 13 of the Exiting-event identification. >> > >> >In FRED spec 5.0, section 9.2 - New VMX Feature: VMX Nested-Exception >> >Support, there is a statement at the end of Exiting-event identification: >> > >> >(The value of this bit is always identical to that of the valid bit of >> >the original-event identification field.) >> > >> >It means that even w/o VMX Nested-Exception support, a VMM already knows >> >if an exception is a nested exception encountered during delivery of >> >another event in an exception caused VM exit (exit reason 0). This is >> >done in KVM through reading IDT_VECTORING_INFO_FIELD and calling >> >vmx_complete_interrupts() immediately after VM exits. >> > >> >vmx_complete_interrupts() simply queues the original exception if there is >> >one, and later the nested exception causing the VM exit could be cancelled >> >if it is a shadow page fault. However if the shadow page fault is caused >> >by a guest page fault, KVM injects it as a nested exception to have guest >> >fix its page table. >> > >> >I will add comments about this background in the next iteration. >> >> is it possible that the CPU encounters an exception and causes VM-exit during >> injecting an __interrupt__? in this case, no __exception__ will be (re-)queued >> by vmx_complete_interrupts(). > >I guess the following case is what you're suggesting: >KVM injects an external interrupt after shadow page tables are nuked. > >vmx_complete_interrupts() are called after each VM exit to clear both >interrupt and exception queues, which means it always pushes the >deepest event if there is an original event. In the above case, the >original event is the external interrupt KVM just tried to inject. in my understanding, your point is: 1. if bit 13 of the Exiting-event identification is set. the original-event identification field should be valid. 2. vmx_complete_interrupts() is done immediately after VM exits and reads original-event identification and reinjects the event there. 3. if KVM injects the exception in exiting-event identification to guest, KVM doesn't need to read the bit 13 because kvm_multiple_exception() is "smart enough" and recognize the exception as nested-exception because if bit 13 is 1, one exception must has been queued in #2. my question is: what if the event in original-event identification is an interrupt e.g., external interrupt or NMI, rather than exception. vmx_complete_interrupts() won't queue an exception, then how can KVM or kvm_multiple_exception() know the exception that caused VM-exit is an nested exception w/o reading bit 13 of the Exiting-event identification?
