Re: [Xen-devel] Rats nest with domain pirq initialisation

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

On Wed, Sep 05, 2018 at 01:39:18PM +0100, Andrew Cooper wrote:
> On 05/09/18 13:25, Jan Beulich wrote:
> >>>> On 05.09.18 at 14:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> >> On 05/09/18 08:24, Jan Beulich wrote:
> >>>>>> On 04.09.18 at 20:44, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>>> The path which blows up is:
> >>>>
> >>>> arch_domain_destroy()
> >>>>   free_domain_pirqs()
> >>>>     unmap_domain_pirq()
> >>>>       irq_deny_access()
> >>>>         rangeset_remove_singleton((d)->irq_caps, i)
> >>> But what IRQ do we find to unmap here? There can't be any that have
> >>> been mapped, when ->irq_caps is still NULL. IOW I don't currently see
> >>> how domain_pirq_to_irq() would legitimately return a positive value at
> >>> this point in time, yet that's what guards the calls to 
> >>> unmap_domain_pirq().
> >> It is pirq 2 which explodes, which is the first of the redundant pirq
> >> structures allocated for legacy routing.
> >>
> >> I'm not sure I understand this code well enough to comment on why
> >> domain_pirq_to_irq() returns a positive value at this point, but I'm
> >> going to go out on a limb and suggest it might be related to our
> >> unnecessary(?) preallocation.
> > I've meanwhile considered this as the reason, too. And iirc the
> > pre-allocation is because guests (including Dom0) bypass some of
> > the setup they would do for non-legacy IRQs. This may have been
> > just a XenoLinux (mis)behavior, but even then I'm not convinced
> > we could easily alter things.
> 
> Bypass which setup?  One way or another they have to bind the irq before
> it can be used, so I still don't see why any structure preallocation is
> needed.  (Reservation of legacy irq numbers, perahps.)

For PIRQs you need to first allocate a PIRQ, then configure it and
bind the PIRQ to an event channel. I have no idea, but it wouldn't
seem that weird that old Dom0 kernels would assume that legacy IRQs
(<16) are already allocated, and just configure and bind them.

Last time I looked when working on legacy PVH support for FreeBSD
pvops Linux would correctly allocate legacy PIRQs before attempting to
bind them, but I haven't looked at classic xenolinux Dom0 kernels.

Roger.

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