Re: buggy emulate_int_real

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-04-12 16:16, Avi Kivity wrote:
> On 04/12/2011 05:12 PM, Serge E. Hallyn wrote:
>> Quoting Avi Kivity (avi@redhat.com):
>>>  On 04/12/2011 10:53 AM, Serge E. Hallyn wrote:
>>>  >Quoting Avi Kivity (avi@redhat.com):
>>>  >>   On 04/09/2011 12:09 AM, Serge E. Hallyn wrote:
>>>  >>   >Hi,
>>>  >>   >
>>>  >>   >at https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/747090, it was
>>>  >>   >found that emulate_int_real() sometimes pushes the wrong eip when doing a
>>>  >>   >int.  Whereas with non-kvm qemu we push the next instruction after the
>>>  >>   >int, with kvm we push the addr of the instruction itself.
>>>  >>   >
>>>  >>
>>>  >>
>>>  >>   The code says:
>>>  >>
>>>  >>       c->src.val = c->eip;
>>>  >>       emulate_push(ctxt, ops);
>>>  >>       rc = writeback(ctxt, ops);
>>>  >>       if (rc != X86EMUL_CONTINUE)
>>>  >>           return rc;
>>>  >>
>>>  >>   which appears to be the address of the next instruction from my
>>>  >>   reading of the code (see how insn_fetch() increments c->eip).
>>>  >
>>>  >Nevertheless removing commits
>>>  >
>>>  >	a92601bb707f6f49fd5563ef3d09928e70cc222e
>>>  >	63995653ade16deacaea5b49ceaf6376314593ac
>>>  >	6e154e56b4d7a6a28c54f0984e13d3f8defc4755
>>>  >
>>>  >changes the eip value being pushed.  If you look at
>>>  >a92601bb707f6f49fd5563ef3d09928e70cc222e, you see:
>>>  >
>>>  >          if (vmx->rmode.vm86_active) {
>>>  >-               vmx->rmode.irq.pending = true;
>>>  >-               vmx->rmode.irq.vector = nr;
>>>  >-               vmx->rmode.irq.rip = kvm_rip_read(vcpu);
>>>  >-               if (kvm_exception_is_soft(nr))
>>>  >-                       vmx->rmode.irq.rip +=
>>>  >-                               vmx->vcpu.arch.event_exit_inst_len;
>>>  >-               intr_info |= INTR_TYPE_SOFT_INTR;
>>>  >-               vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr_info);
>>>  >-               vmcs_write32(VM_ENTRY_INSTRUCTION_LEN, 1);
>>>  >-               kvm_rip_write(vcpu, vmx->rmode.irq.rip - 1);
>>>  >+               if (kvm_inject_realmode_interrupt(vcpu, nr) != EMULATE_DONE)
>>>  >+                       kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>>>  >                  return;
>>>  >          }
>>>  >
>>>  >but kvm_inject_realmode_interrupt() does not appear to increment
>>>  >vmx->rmode.irq.rip anywhere, as the code being replaced does.
>>>
>>>  Ah, I see now.  There are two cases, hard interrupt and soft
>>>  interrupts.  I guess hard interrupts are handled fine, and the
>>>  failing case is
>>>
>>>    guest executes INTn instruction in guest mode
>>>    vmx intercepts a page fault (say due to access to the IDT or the stack)
>>>    kvm notes that a soft interrupt was in progress (vmx_complete_interrupts)
>>>    kvm handles the exception
>>>    reinject the interrupt while reentering the guest
>>>
>>>  so we do need something like
>>>
>>>     if (soft)
>>>         vcpu->arch.emulate_ctxt.eip += inst_len;
>>>
>>>  in kvm_inject_realmode_interrupt().
>>
>> Oops, right.  Disregard last email pls :)
>>
>> So is 'kvm_exception_is_soft(irq)' a reliable check?
>>
> 
> No, need to check vcpu->arch.interrupt.soft instead.  Not sure about 
> kvm_exception_is_soft().  Jan?

Jumping late on this, I don't understand the question. Reliable /wrt what?

Jan

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