On Tue, Nov 02, 2021, Vitaly Kuznetsov wrote: > Sean Christopherson <seanjc@xxxxxxxxxx> writes: > > But that mess is a red herring, the test fails with the same signature with APICv=1 > > if the STI is replaced by PUSHF+BTS+POPFD (to avoid the STI shadow). We all missed > > this key detail from Vitaly's report: > > > > SINGLE_STEP[1]: exit 8 exception 1 rip 0x402a25 (should be 0x402a27) dr6 0xffff4ff0 (should be 0xffff4ff0) > > ^^^^^^ > > > > Exit '8' is KVM_EXIT_SHUTDOWN, i.e. the arrival of the IRQ hosed the guest because > > the test doesn't invoke vm_init_descriptor_tables() to install event handlers. > > The "exception 1" shows up because the run page isn't sanitized by the test, i.e. > > it's stale data that happens to match. > > > > So I would fully expect this test to fail with AVIC=1. The problem is that > > KVM_GUESTDBG_BLOCKIRQ does absolutely nothing to handle APICv interrupts. And > > even if KVM does something to fudge that behavior in the emulated local APIC, the > > test will then fail miserably virtual IPIs (currently AVIC only). > > FWIW, the test doesn't seem to fail on my AMD EPYC system even with "avic=1" ... Huh. Assuming the IRQ is pending in the vIRR and KVM didn't screw up elsewhere, that seems like a CPU AVIC bug. #DBs have priority over IRQs, but single-step #DBs are trap-like and KVM (hopefully) isn't injecting a #DB, so a pending IRQ should be taken on the current instruction in the guest when executing VMRUN with guest.EFLAGS.IF=1,TF=1 since there will be a one-instruction delay before the single-step #DB kicks in.
