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

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Mon, Nov 01, 2021, Maxim Levitsky wrote:
>> On Mon, 2021-11-01 at 16:43 +0100, Vitaly Kuznetsov wrote:
>> > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
>> > 
>> > > On 11/08/21 14:29, Maxim Levitsky wrote:
>> > > > Modify debug_regs test to create a pending interrupt
>> > > > and see that it is blocked when single stepping is done
>> > > > with KVM_GUESTDBG_BLOCKIRQ
>> > > > 
>> > > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
>> > > > ---
>> > > >   .../testing/selftests/kvm/x86_64/debug_regs.c | 24 ++++++++++++++++---
>> > > >   1 file changed, 21 insertions(+), 3 deletions(-)
>> > > 
>> > > I haven't looked very much at this, but the test fails.
>> > > 
>> > 
>> > Same here,
>> > 
>> > the test passes on AMD but fails consistently on Intel:
>> > 
>> > # ./x86_64/debug_regs 
>> > ==== Test Assertion Failure ====
>> >   x86_64/debug_regs.c:179: run->exit_reason == KVM_EXIT_DEBUG && run->debug.arch.exception == DB_VECTOR && run->debug.arch.pc == target_rip && run->debug.arch.dr6 == target_dr6
>> >   pid=13434 tid=13434 errno=0 - Success
>> >      1	0x00000000004027c6: main at debug_regs.c:179
>> >      2	0x00007f65344cf554: ?? ??:0
>> >      3	0x000000000040294a: _start at ??:?
>> >   SINGLE_STEP[1]: exit 8 exception 1 rip 0x402a25 (should be 0x402a27) dr6 0xffff4ff0 (should be 0xffff4ff0)
>> > 
>> > (I know I'm late to the party).
>> 
>> Well that is strange. It passes on my intel laptop. Just tested 
>> (kvm/queue + qemu master, compiled today) :-(
>> 
>> It fails on iteration 1 (and there is iteration 0) which I think means that we
>> start with RIP on sti, and get #DB on start of xor instruction first (correctly), 
>> and then we get #DB again on start of xor instruction again?
>> 
>> Something very strange. My laptop has i7-7600U.
>
> I haven't verified on hardware, but my guess is that this code in vmx_vcpu_run()
>
> 	/* When single-stepping over STI and MOV SS, we must clear the
> 	 * corresponding interruptibility bits in the guest state. Otherwise
> 	 * vmentry fails as it then expects bit 14 (BS) in pending debug
> 	 * exceptions being set, but that's not correct for the guest debugging
> 	 * case. */
> 	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
> 		vmx_set_interrupt_shadow(vcpu, 0);
>
> interacts badly with APICv=1.  It will kill the STI shadow and cause the IRQ in
> vmcs.GUEST_RVI to be recognized when it (micro-)architecturally should not.  My
> head is going in circles trying to sort out what would actually happen.  Maybe
> comment out that and/or disable APICv to see if either one makes the test pass?
>

Interestingly,

loading 'kvm-intel' with 'enable_apicv=0' makes the test pass, however,
commenting out "vmx_set_interrupt_shadow()" as suggested gives a
different result (with enable_apicv=1):

# ./x86_64/debug_regs 
==== Test Assertion Failure ====
  x86_64/debug_regs.c:179: run->exit_reason == KVM_EXIT_DEBUG && run->debug.arch.exception == DB_VECTOR && run->debug.arch.pc == target_rip && run->debug.arch.dr6 == target_dr6
  pid=16352 tid=16352 errno=0 - Success
     1	0x0000000000402b33: main at debug_regs.c:179 (discriminator 10)
     2	0x00007f36401bd554: ?? ??:0
     3	0x00000000004023a9: _start at ??:?
  SINGLE_STEP[1]: exit 9 exception -2147483615 rip 0x1 (should be 0x4024d9) dr6 0xffff4ff0 (should be 0xffff4ff0)

this is a fairly old "Intel(R) Xeon(R) CPU E5-2603 v3".

-- 
Vitaly