Re: [Xen-devel] [PATCH] xen/events: remove event handling recursion detection

[JÃrgen GroÃ]

On 04.11.19 15:35, Jan Beulich wrote:
> On 04.11.2019 14:58, Juergen Gross wrote:
> > __xen_evtchn_do_upcall() contains guards against being called
> > recursively. This mechanism was introduced in the early pvops times
> > (kernel 2.6.26) when there were still Xen versions around not honoring
> > disabled interrupts for sending events to pv guests.
> >
> > This was changed in Xen 3.0, which is much older than any Xen version
> > supported by the kernel, so the recursion detection can be removed.
>
> Would you mind pointing out which exact change(s) this was(were)?

Linux kernel: 229664bee6126e01f8662976a5fe2e79813b77c8
Xen: d8263e8dbaf5ef1445bee0662143a0fcb6d43466

> It had always been my understanding that the recursion detection
> was mainly to guard against drivers re-enabling interrupts
> transiently in their handlers (which in turn may no longer be an
> issue in modern Linux kernels).

This would have been doable with a simple bool. The more complex
xchg based logic was IMO for recursion detection at any point.


Juergen