Re: [Xen-devel] [PATCH] X86/vmx: Dump PIR and vIRR before ASSERT()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.02.17 at 07:32, <chao.gao@xxxxxxxxx> wrote:
> On Tue, Feb 07, 2017 at 03:04:32AM -0700, Jan Beulich wrote:
>>>>> On 07.02.17 at 07:48, <xuquan8@xxxxxxxxxx> wrote:
>>> Some comment from QEMU/KVM code, in <linux-kernel>/arch/x86/kvm/lapic.c,
>>> 
>>> int kvm_lapic_find_highest_irr(struct kvm_vcpu *vcpu)
>>> {
>>>     /* This may race with setting of irr in __apic_accept_irq() and
>>>      * value returned may be wrong, but kvm_vcpu_kick() in __apic_accept_irq
>>>      * will cause vmexit immediately and the value will be recalculated
>>>      * on the next vmentry.
>>>      */
>>> ...
>>> }
>>> 
>>> I am afraid, there may be a similar race with setting of vIRR..
>>
>>I think this is unrelated: If another interrupt came in, the highest
>>set bit in vIRR can only increase. Plus pt_update_irq(), before
>>returning, calls vlapic_set_irq(), which ought to result in pt_vector's
>>vIRR bit to be set (either directly or via setting its PIR bit). I.e. the
>>highest priority interrupt should still have a vector >= pt_vector.
> 
> I have noticed that pt_vector was 0x38 and the hightest vector was 0x30
> when the assertion failed. In the process of debugging pt_update_irq() 
> when I was working on another feature, I noticed that 0x30 is always 
> the vector of IRQ2. I suspect that the source of the periodic timer interrupt
> is not lapic. If that, pt_update_irq() reads the vioapic entry twice,
> one in hvm_isa_irq_assert() and the other in pt_irq_vector().
> If guest changed the content in viopaic entry between the two read
> (ie. change the vector from 0x30 to 0x38), the assertion would fail.
> Do you think it is a reasonable explanation?

IRQ2? If this was going via the PIC, that would be impossible, as
that's the cascade pin. IRQ0, otoh, normally arrives at pin2 of the
IO-APIC (and we follow this model - see tools/libacpi/build.c). How
did you determine that 0x30 is the vector of IRQ2?

Jan


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