Re: [PATCH RFC V9 4/5] xen, libxc: Request page fault injection via libxc

[Andrew Cooper <andrew.cooper3@citrix.com>]

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