Re: [Xen-devel] RFC Userspace hypercalls

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.01.16 at 17:20, <andrew.cooper3@xxxxxxxxxx> 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


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