Re: [Xen-devel] [PATCH] Remove a set operation for VCPU_KICK_SOFTIRQ when post interrupt to vm.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.09.15 at 10:10, <yang.z.zhang@xxxxxxxxx> wrote:
> Liuqiming (John) wrote on 2015-09-08:
>> Ok, I will try to explain, correct me if I got anything wrong:
>> 
>> The problem here is not interrupts lost but interrupts not delivered in
>> time.
>> 
>> there are basically two path to inject an interrupt into VM  (or vCPU to
>> another vCPU):
>> Path 1, the traditional way:
>>        1) set bit  in vlapic IRR field which represent an interrupt,
>>        then kick vcpu 2) a VCPU_KICK_SOFTIRQ softirq raised 3) if
>>        VCPU_KICK_SOFTIRQ bit not set, then set it, otherwise return and
>>        do nothing 4) send an EVENT_CHECK_VECTOR IPI  to target vcpu 5)
>>        target vcpu will VMEXIT due to EXIT_REASON_EXTERNAL_INTERRUPT 6)
>>        the interrupt handler basically do nothing 7) interrupt in IRR
>>        will be evaluated 8) VCPU_KICK_SOFTIRQ will be cleared when
>>        do_softirq 9) there will be an interrupt inject into vcpu when
>>        VMENTRY
>> Path 2, the Posted-interrupt way (current logic):
>>        1) set bit in posted-interrupt descriptor which represent an
>>        interrupt 2) if VCPU_KICK_SOFTIRQ bit not set, then set it,
>>        otherwise return and do nothing 3) send an
>>        POSTED_INTR_NOTIFICATION_VECTOR IPI to target vcpu 4) if target
>>        vcpu in non-ROOT mode it will receive the interrupt
>> immediately otherwise interrupt will be injected when VMENTRY
>> 
>> As the first operation in both path is setting a interrupt represent
>> bit, so no interrupts will lost.
>> 
>> The issue is:
>> in path 2, the first interrupt will cause VCPU_KICK_SOFTIRQ set to 1,
>> and unless a VMEXIT occured or somewhere called do_softirq directly,
>> VCPU_KICK_SOFTIRQ will not cleared, that will make the later interrupts
>> injection  ignored at step 2),
>> which will delay irq handler process in VM.
>> 
>> And because path 2 set VCPU_KICK_SOFTIRQ to 1, the kick vcpu logic in
>> path 1 will also return in step 3),
>> which make this vcpu only can handle interrupt when some other reason
>> cause VMEXIT.
> 
> Nice catch! But without set the softirq, the interrupt delivery also will be 
> delay.

Right. The patch as it stands, while improving performance, breaks
correctness (by - theoretically - deferring delivery of the interrupt
indefinitely). Yet there certainly is room for improvement, by finding
a correct condition for suppressing the softirq.

Jan


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