Re: [Xen-devel] ioreq process conflict when EVTCHNOP_bind_interdomain hypercall and vcpu pio occur concurrently

[Liuyongan <liuyongan@xxxxxxxxxx>]

If Qemu calls xc_evtchn_bind_interdomain to allocate an free port for each 
vcpu, 
it will receive empty io-request triggered by xen. I say it empty as the ioreq 
in
shared page is not filled. So qemu can simply skip the first ioreq of each vcpu 
by
a pervcpu flag.

You should not skip the first one by check ioreq->state , as the shared page may
be modified by real vm trigged io req, so ioreq->state becomes REQUEST READY.

Any bugs here?

Liuyongan
2014-11-14


> -----Original Message-----
> From: Liuyongan
> Sent: Tuesday, November 11, 2014 8:45 PM
> To: 'xen-devel@xxxxxxxxxxxxx'
> Cc: 'JBeulich@xxxxxxxx'; Shanhaitao (Tony); Huangzhichao; zhangyuexi;
> Fanhenglong; Qianhuibin
> Subject: ioreq process conflict when EVTCHNOP_bind_interdomain hypercall
> and vcpu pio occur concurrently
> 
> I wonder if it is necessary for xen to trigger the event channel pending when
> the port related a vcpu port io.
> 
> Suppose a scenario as follows:
> 
> 1)  Qemu make a hypercall using codes:
>      for (i = 0; i < smp_cpus; i++) {
>         rc = xc_evtchn_bind_interdomain(state->xce_handle, xen_domid,
> 
> xen_vcpu_eport(state->shared_page, i));
>         if (rc == -1) {
>             fprintf(stderr, "bind interdomain ioctl(shared_page) error %d\n",
> errno);
>             return -1;
>         }
>         state->ioreq_local_port[i] = rc;
>         ...
>      }
> 
> 2)  Xen do_event_channel_op allocate a free port and call evtchn_set_pending
> to trigger a evtchn event.
> 
> 3)  Qemu enters main_loop and begin the evtchn event (pio event).
> 
> 4)  The vcpus of a vm begin to trigger real pio exit,  and this ioreq_t will
> conflict with the one triggered in step 2.
> 
> This will certainly cause failures of real port io.
> 
> Does anyone here have any suggestions?
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