Re: [PATCH v3 6/6] KVM: selftests: test KVM_GUESTDBG_BLOCKIRQ

[Maxim Levitsky]

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