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

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Message-ID: <acaf58cb-47f2-7e7e-f25d-ff83ae8a8066@suse.com>
Date:   Mon, 4 Nov 2019 16:09:38 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/events: remove event handling recursion
 detection

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