Re: RFC Userspace hypercalls

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

On 06/01/16 16:49, Jan Beulich wrote:
>>>> On 06.01.16 at 17:38, <andrew.cooper3@citrix.com> wrote:
>> On 06/01/16 16:31, Jan Beulich wrote:
>>>>>> On 06.01.16 at 15:44, <andrew.cooper3@citrix.com> wrote:
>>>> We did have an internal request for an HVM guest userspace netfront
>>>> driver to be able to use evntchnop calls directly.
>>> And this can't be accomplished using the evtchn and/or privcmd
>>> drivers?
>> It can, and I don't believe the worry about extra overhead is well
>> placed.  (There many areas of lower hanging fruit in this specific case).
>>
>> However, a userspace backend isn't in principle a bad idea.
> Backend? Earlier you said frontend.

I did mean frontend, but it really doesn't matter as far as this is
concerned.

> Nor can I see how using the evtchn/privcmd devices would preclude that.

They don't.  I didn't imply that they would.

> After all their purpose is to avoid having to expose hypercalls directly.

Only one purpose.  Another purpose is to enforce separation between
processes, and handle allocation of global resources.

In a dedicated utility VM, where all components are trusted, none of
these reasons have as much weight as they do in a general purpose OS,
and there is a valid argument to be made for favouring performance over
isolation.

I am not suggesting that userspace hypercalls would make orders of
magnitude difference, but they would make some difference, and allow a
Xen domain to have a more rDMA-like approach, if it chooses.

~Andrew