Re: [Xen-devel] [PATCH v8 01/17] VT-d Posted-intterrupt (PI) design

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Monday, October 12, 2015 6:42 PM
> To: Wu, Feng; xen-devel@xxxxxxxxxxxxx
> Cc: Tian, Kevin; Zhang, Yang Z; Jan Beulich; Keir Fraser; George Dunlap
> Subject: Re: [PATCH v8 01/17] VT-d Posted-intterrupt (PI) design
> 
> On 12/10/15 09:54, Feng Wu wrote:
> > Add the design doc for VT-d PI.
> >
> > CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> > CC: Yang Zhang <yang.z.zhang@xxxxxxxxx>
> > CC: Jan Beulich <jbeulich@xxxxxxxx>
> > CC: Keir Fraser <keir@xxxxxxx>
> > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  docs/misc/vtd-pi.txt | 332
> +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 332 insertions(+)
> >  create mode 100644 docs/misc/vtd-pi.txt
> >
> There appears to be a race condition here.  Consider the following setup.
> 
> VCPU A with PID A
> VCPU B with PID B
> 
> Both VCPUs have Posted Interrupts set up and devices assigned.
> 
> VCPU A is running in non-root mode on CPU X, and VCPU B is on the
> blocked list for CPU X.  i.e. Both PID A and B have the same ndst and nv.

vCPUA is running in non-root mode, so its nv is 'posted_intr_vector', and
vCPUS is blocked, so its nv is 'pi_wakeup_vector', the NV is different, so
the situation you describe cannot happen.

Thanks,
Feng

> 
> An interrupt arrives to PID B, setting B.ON and sending a notification
> vector for CPU X.
> 
> At this point, according to the spec, CPU X will collect interrupt
> information from PID A and inject any vectors locally.
> 
> This this accurate?  If so, what causes a #VMEXIT to occur and for Xen
> to service the blocked list on CPU X?
> 
> ~Andrew

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