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

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Date:   Mon, 4 Nov 2019 16:19:43 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Jürgen Groß <jgross@...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 04.11.2019 16:09, Jürgen Groß wrote:
> 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

Are you sure about the latter, touching only header files underneath
xen/, and there mostly public interface ones?

>> 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.

Well, the respective XenoLinux c/s 13098 has no mention of this, i.e.
it simply leaves open what the actual reason was:

"[LINUX] Disallow nested event delivery.

 This eliminates the risk of overflowing the kernel stack and is a
 reasonable policy given that we have no concept of priorities among
 event sources."

Jan