From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH V5 08/12] xen/vm_event: Check for
 VM_EVENT_FLAG_DUMMY only in Debug builds
Date: Fri, 13 Feb 2015 23:00:24 +0000
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To: Tamas K Lengyel <tamas.lengyel@zentific.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>, "wei.liu2@citrix.com" <wei.liu2@citrix.com>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Tim Deegan <tim@xen.org>, Steven Maresca <steve@zentific.com>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>, Jan Beulich <jbeulich@suse.com>, "Dong, Eddie" <eddie.dong@intel.com>, Andres Lagar-Cavilla <andres@lagarcavilla.org>, Jun Nakajima <jun.nakajima@intel.com>, "rshriram@cs.ubc.ca" <rshriram@cs.ubc.ca>, Keir Fraser <keir@xen.org>, Daniel De Graaf <dgdegra@tycho.nsa.gov>, "yanghy@cn.fujitsu.com" <yanghy@cn.fujitsu.com>, Ian Jackson <ian.jackson@eu.citrix.com>
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On 13/02/2015 22:53, Tamas K Lengyel wrote:
> On Fri, Feb 13, 2015 at 11:48 PM, Tamas K Lengyel
> <tamas.lengyel@zentific.com> wrote:
>> On Fri, Feb 13, 2015 at 9:14 PM, Andrew Cooper
>> <andrew.cooper3@citrix.com> wrote:
>>> On 13/02/15 16:33, Tamas K Lengyel wrote:
>>>> The flag is only used for debugging purposes, thus it should be only checked
>>>> for in debug builds of Xen.
>>>>
>>>> Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
>>> What is the purpose of the dummy flag?  I would have thought it would be
>>> more dangerous to accidentally process a dummy response in a non-debug Xen.
>>>
>>> ~Andrew
>> I honestly have no idea what was the real use-case for it. It is a way
>> to signal to Xen to just remove the reponse from the ring without
>> doing anything else with it so I figured it might be handy when
>> debugging a toolstack.
>>
>> Tamas
> As for processing it on a non-debug Xen: I think it's safe as the
> response would just be handled according to the other flags that are
> set in the response, like unpausing the vCPU that triggered the event.
> IMHO its a wasteful check on production systems.

If it is not useful then discard it completely (seeing as the entire
system is being overhauled).  I can't see any purpose in having a method
of telling Xen to ignore a request placed on the ring.  If the toolstack
wishes Xen not to do anything then it should not have put a request on
the ring in the first place.

~Andrew