[Qemu-devel] Re: backdoor [Was: Re: [PATCH 00/17] [RFC] static instrumentation]

[Paolo Bonzini]

On 10/25/2010 12:54 PM, Lluís wrote:
> * Backdoor channels need to provide arguments.
> * It's better to provide the same mechanism for both *-user and softmmu
>    (otherwise the application to simulate or the interposed librariy
>    should be compiled differently on every case).

You can add the syscall and, if it returns with ENOSYS, fall back to 
MMIO/PIO (you don't really need a special driver, only some chmod since 
BARs are accessible from /sys) or watchpoint/breakpoint.

> * Some applications are not fully simulated, but just some pre-defined
>    loops are, such that I would run the application (on KVM mode if
>    possible) until the interesting loop starts executing, switch to
>    simulation mode, simulate an arbitrary number of loops, switch back to
>    non-simulated mode, run until another specific loop or iteration is
>    reached, repeat the process again.
>    This obviously requires some application modification to insert the
>    communication points, unless these can be identified using symbol
>    interposition (e.g., calls to the OpenMP runtime to simulate certain
>    parallel loops).

Switching between KVM and TCG is not possible at the moment, even though 
in principle it should be feasible for non-SMP guests.

Paolo