Re: RFC Userspace hypercalls

[Andrew Cooper <andrew.cooper3@citrix.com>]

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`.

~Andrew