On Tue, 2021-11-02 at 18:45 +0000, Sean Christopherson wrote: > 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" ... Its because AVIC is inhibited for many reasons. In this test x2apic is used, and having x2apic in CPUID inhibits AVIC. > > 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. > We could inhibit AVIC/APICv when KVM_GUESTDBG_BLOCKIRQ is in use, I'll send patch for this soon. Thanks a lot for finding out what is going on! Best regards, Maxim Levitsky
