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

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.

Razvan Cojocaru

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