Re: Event delivery and "domain blocking" on PVHv2

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 22, 2020 at 06:20:34PM +0200, Jan Beulich wrote:
> On 22.06.2020 18:09, Roger Pau Monné wrote:
> > On Mon, Jun 22, 2020 at 05:31:00PM +0200, Martin Lucina wrote:
> >> On 2020-06-22 15:58, Roger Pau Monné wrote:
> >>> On Mon, Jun 22, 2020 at 12:58:37PM +0200, Martin Lucina wrote:
> >>>> Aha! Thank you for pointing this out. I think you may be right, but
> >>>> this
> >>>> should be possible without doing the demuxing in interrupt context.
> >>>
> >>> If you don't do the demuxing in the interrupt context (ie: making the
> >>> interrupt handler a noop), then you don't likely need such interrupt
> >>> anyway?
> >>
> >> I need the/an interrupt to wake the VCPU from HLT state if we went to 
> >> sleep.
> >>
> >>>
> >>>> How about this arrangement, which appears to work for me; no hangs I
> >>>> can see
> >>>> so far and domU survives ping -f fine with no packet loss:
> >>>>
> >>>> CAMLprim value
> >>>> mirage_xen_evtchn_block_domain(value v_deadline)
> >>>> {
> >>>>     struct vcpu_info *vi = VCPU0_INFO();
> >>>>     solo5_time_t deadline = Int64_val(v_deadline);
> >>>>
> >>>>     if (solo5_clock_monotonic() < deadline) {
> >>>>         __asm__ __volatile__ ("cli" : : : "memory");
> >>>>         if (vi->evtchn_upcall_pending) {
> >>>>             __asm__ __volatile__ ("sti");
> >>>>         }
> >>>>         else {
> >>>>             hypercall_set_timer_op(deadline);
> >>>
> >>> What if you set a deadline so close that evtchn_upcall_pending gets
> >>> set by Xen here and the interrupt is injected? You would execute the
> >>> noop handler and just hlt, and could likely end up in the same blocked
> >>> situation as before?
> >>
> >> Why would an interrupt be injected here? Doesn't the immediately preceding
> >> "cli" disable that?
> > 
> > Well, I mean between the sti and the hlt instruction.
> 
> When EFLAGS.IF was clear before STI, then the first point at which
> an interrupt can get injected is when HLT is already executed (i.e.
> to wake from this HLT). That's the so called "STI shadow".

Oh, so then what Martin does seems to be fine, as there's no race, by
the fact that evtchn_upcall_pending is checked with interrupts
disabled.

Thanks, Roger.