From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH RFC V9 4/5] xen,
 libxc: Request page fault injection via libxc
Date: Wed, 10 Sep 2014 09:48:07 +0100
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To: Razvan Cojocaru <rcojocaru@bitdefender.com>, Tamas K Lengyel <tamas.lengyel@zentific.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Jun Nakajima <jun.nakajima@intel.com>, "Dong,
	Eddie" <eddie.dong@intel.com>, George Dunlap <dunlapg@umich.edu>, Tim Deegan <tim@xen.org>, Jan Beulich <JBeulich@suse.com>, xen-devel <xen-devel@lists.xenproject.org>, Ian Jackson <ian.jackson@eu.citrix.com>
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On 10/09/2014 09:09, Razvan Cojocaru wrote:
> On 09/09/2014 09:38 PM, Tamas K Lengyel wrote:
>>     > But ultimately, as Tim said, you're basically just *hoping* that it
>>     > won't take too long to happen to be at the hypervisor when the proper
>>     > condition happens.  If the process in question isn't getting many
>>     > interrupts, or is spending the vast majority of its time in the
>>     > kernel, you may end up waiting an unbounded amount of time to be able
>>     > to "catch" it in user mode.  It seems like it would be better to find
>>     > a reliable way to trap on the return into user mode, in which case you
>>     > wouldn't need to have a special "wait for this complicated event to
>>     > happen" call at all, would you?
>>
>>     Indeed, but it is assumed that the trap injection request is being made
>>     by the caller in the proper context (when it knows that the condition
>>     will be true sooner rather than later).
>>
>>
>> How is it known that the condition will be true soon? Some more
>> information on what you consider 'proper context' would be valuable.
> It's actually pretty simple for us: the application always requests an
> injection when the guest is already in the address space of the
> interesting application, and in user mode.

Does this mean that you always request a pagefault as a direct result of
a mem_event, when the vcpu is in blocked the correct context?

If so, how about extending the mem_event response mechanism with
trap/fault information?

~Andrew