Re: [Xen-devel] [PATCH v6 15/18] vmx: Properly handle notification event when vCPU is running

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.09.15 at 11:36, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Tuesday, September 08, 2015 5:31 PM
>> To: Wu, Feng
>> Cc: Andrew Cooper; Tian, Kevin; Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx; Keir
>> Fraser
>> Subject: RE: [PATCH v6 15/18] vmx: Properly handle notification event when
>> vCPU is running
>> 
>> >>> On 08.09.15 at 11:23, <feng.wu@xxxxxxxxx> wrote:
>> 
>> >>
>> >> Now that you say it, this looks even more odd: Why would you need
>> >> to raise that softirq if the only way to come here is via the triggering
>> >> in __vmx_deliver_posted_interrupt() (which already raised that
>> >> softirq)? I suppose I must be missing something...
>> >
>> > Before VT-d PI, __vmx_deliver_posted_interrupt() is the only way
>> > to trigger interrupt with vector ' posted_intr_vector ', but after
>> > introducing VT-d PI, VT-d hardware can issue the interrupt with
>> > that vector as well. In __vmx_deliver_posted_interrupt(), it set
>> > the softirqs, the reason of which is described in the comments
>> > of pi_notification_interrupt(), however, I need do the same thing
>> > when VT-d hardware issue the interrupt, so pi_notification_interrupt()
>> > is the proper place to do it.
>> 
>> But again - my main concern is about the !iommu_intpost case: What
>> good does the raising of the softirq there? (As a general remark -
>> please, when you add code to support a new feature, don't just
>> think about the case where the new feature is available in hardware,
>> but also about the case where it's not. While over time the set of
>> systems lacking the feature will likely decrease, initially it may be the
>> vast majority of systems Xen gets run on which would get penalized.)
> 
> No problem. Two solutions:
> #1, Register pi_notification_interrupt when iommu_intpost and still
> use event_check_interrupt in the !iommu_intpost case.
> #1, Use pi_notification_interrupt() for both iommu_intpost and
> !iommu_intpost, add the following check it:
>       if ( iommu_intpost )
>               raise_softirq(VCPU_KICK_SOFTIRQ);
> 
> Which one do you prefer?

#1 ;-) (the earlier one, to avoid you guessing)

Jan


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