From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-7555-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id A8E6C98605B for ; Wed, 15 Jul 2020 17:09:51 +0000 (UTC) Date: Wed, 15 Jul 2020 19:09:39 +0200 From: Cornelia Huck Message-ID: <20200715190939.60b562f9.cohuck@redhat.com> In-Reply-To: <871rlcybni.fsf@linaro.org> References: <87r1tdydpz.fsf@linaro.org> <20200715114855.GF18817@stefanha-x1.localdomain> <877dv4ykin.fsf@linaro.org> <20200715154732.GC47883@stefanha-x1.localdomain> <871rlcybni.fsf@linaro.org> MIME-Version: 1.0 Subject: Re: [virtio-dev] On doorbells (queue notifications) Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable To: Alex =?UTF-8?B?QmVubsOpZQ==?= Cc: Stefan Hajnoczi , virtio-dev@lists.oasis-open.org, Zha Bin , Jing Liu , Chao Peng , Jan Kiszka List-ID: On Wed, 15 Jul 2020 17:40:33 +0100 Alex Benn=C3=A9e wrote: > I was also wondering about the efficiency of doorbells/notifications the > other way. AFAIUI for both PCI and MMIO only a single write is required > to the notify flag which causes a trap to the hypervisor and the rest of > the processing. The hypervisor doesn't have the cost multiple exits to > read the guest state although it obviously wants to be as efficient as > possible passing the data back up to what ever is handling the backend > of the device so it doesn't need to do multiple context switches. >=20 > Has there been any investigation into other mechanisms for notifying the > hypervisor of an event - for example using a HYP call or similar > mechanism? For ccw devices, we do a 'diagnose' call (which is basically a hypercall). It has some parameters (including the queue the guest is notifying for), so the host already has that information when the exit happens. > My gut tells me this probably doesn't make any difference as a trap to > the hypervisor is likely to cost the same either way because you still > need to save the guest context before actioning something but it would > be interesting to know if anyone has looked at it. Perhaps there is a > benefit in partitioned systems where core running the guest can return > straight away after initiating what it needs to internally in the > hypervisor to pass the notification to something that can deal with it? I guess that depends mostly upon whether there is further interaction needed, or if the guest can give the host all needed info in one go. --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org