On Tue, Oct 09, 2012 at 06:55:58PM +0200, Paolo Bonzini wrote:
Il 09/10/2012 18:19, Aurelien Jarno ha scritto:
Instead of calling the MMU helper with an additional argument (7), and
then jump back (8) to the next code (4), what about pushing the address
of the next code (4) on the stack and use a jmp instead of the call. In
that case you don't need the extra argument to the helpers.
Maybe it wasn't very clear. This is based on the fact that call is
basically push %rip + jmp. Therefore we can fake the return address by
putting the value we want, here the address of the next code. This mean
that we don't need to pass the extra argument to the helper for the
return address, as GET_PC() would work correctly (it basically reads the
return address on the stack).
For other architectures, it might not be a push, but rather a move to
link register, basically put the return address where the calling
convention asks for.
OTOH I just realized it only works if the end of the slow path (moving
the value from the return address to the correct register). It might be
something doable.
Branch predictors will not oldschool tricks like this one. :)
Given it is only used in the slow path (ie the exception more than the
rule), branch prediction isn't that important there.