From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: RFC Userspace hypercalls
Date: Wed, 6 Jan 2016 16:20:12 +0000
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To: Jan Beulich <JBeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>, Tim Deegan <tim@xen.org>, Xen-devel List <xen-devel@lists.xen.org>
List-Id: xen-devel@lists.xenproject.org

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