From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: RFC Userspace hypercalls Date: Wed, 6 Jan 2016 17:06:11 +0000 Message-ID: <568D4983.5030907@citrix.com> References: <568CFE3A.8080505@citrix.com> <568D2F4002000078000C3F7E@prv-mh.provo.novell.com> <568D2860.3010807@citrix.com> <568D4F6502000078000C418C@prv-mh.provo.novell.com> <568D42F2.6070900@citrix.com> <568D53B202000078000C41CC@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <568D53B202000078000C41CC@prv-mh.provo.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: Keir Fraser , TimDeegan , Xen-devel List List-Id: xen-devel@lists.xenproject.org On 06/01/16 16:49, Jan Beulich wrote: >>>> On 06.01.16 at 17:38, wrote: >> On 06/01/16 16:31, Jan Beulich wrote: >>>>>> On 06.01.16 at 15:44, 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