>>> On 06.01.16 at 17:20, <andrew.coop...@citrix.com> wrote: > On 06/01/16 16:09, Jan Beulich wrote: >> >>>>> For PV guests, I propose that userspace hypercalls get implemented with >>>>> the int $0x82 path exclusively. i.e. enabling userspace hypercalls >>>>> causes the hypercall page writing logic to consider the guest a ring1 >>>>> kernel, and the int $0x82 entrypoint suitably delegates between a >>>>> regular hypercall and a compat hypercall. >>>> With int $0x82 being the primary hypercall path for 32-bit guests, >>>> I'd be concerned of any code addition, especially that of further >>>> conditionals. >>> The overhead of one extra conditional in the hypercall path is lost in >>> the noise, compared to the overhead of the task switch itself. >> Task switch? On the hypercall path? > > Apologies - I meant the context switch caused by `int $0x82`.
I don't think a software interrupt is unilaterally on all hardware so slow that a mispredicted branch would be completely unnoticable. Jan _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
