Re: [Xen-devel] [PATCH V5 08/12] xen/vm_event: Check for VM_EVENT_FLAG_DUMMY only in Debug builds

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Sat, Feb 14, 2015 at 12:00 AM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/02/2015 22:53, Tamas K Lengyel wrote:
>> On Fri, Feb 13, 2015 at 11:48 PM, Tamas K Lengyel
>> <tamas.lengyel@xxxxxxxxxxxx> wrote:
>>> On Fri, Feb 13, 2015 at 9:14 PM, Andrew Cooper
>>> <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxxxxxx>
>>>> 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

Ack.

Tamas

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