From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF3A5C63777 for ; Mon, 30 Nov 2020 16:49:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2439F2073C for ; Mon, 30 Nov 2020 16:49:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="uFW1GYkg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728975AbgK3Qta (ORCPT ); Mon, 30 Nov 2020 11:49:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41184 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728794AbgK3Qt3 (ORCPT ); Mon, 30 Nov 2020 11:49:29 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4B2D0C0613CF; Mon, 30 Nov 2020 08:48:49 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=Mime-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=YnMmKVw/+QW8i5WxQRj92p1rcbtLV7skjsUNsfU2Few=; b=uFW1GYkgchozj/Rw0iBPcTFucK PQnYyxdR2yt1c2cFYIN42ln0REPT47DBRBO1xfGs9BLOvp2HtQYBKBhEMO89yYdTU3dC3eqW218vG OpYtNsJUafis47xELDaszxGAxT0SaWvgdG8HePSzXkqsm6Hpp3s+9s8BgOme8GnlX18WrKZdq4Z/2 ySjxB4QN0VB/rvO0PUDJVZloImxTWWvZnemGbyapYevOwxJWwY+1OXF3BxaU1YfHB1h3xMa+iDcVB xFu75zQM5ommS9e9/V5nikM4z/vMUa3JlOINNn1MLS0f0A4aU5h5s6WtfJ4FJmKF3MTYpjSq7O9oh 9Dr+cX1w==; Received: from 54-240-197-231.amazon.com ([54.240.197.231] helo=freeip.amazon.com) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kjmLl-0005YK-6m; Mon, 30 Nov 2020 16:48:29 +0000 Message-ID: <315ea414c2bf938978f7f2c0598e80fa05b4c07b.camel@infradead.org> Subject: Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd From: David Woodhouse To: Joao Martins , Ankur Arora Cc: Boris Ostrovsky , Paolo Bonzini , Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 30 Nov 2020 16:48:26 +0000 In-Reply-To: <0b9d3901-c10b-effd-6278-6afd1e95b09e@oracle.com> References: <20190220201609.28290-1-joao.m.martins@oracle.com> <20190220201609.28290-12-joao.m.martins@oracle.com> <874d1fa922cb56238676b90bbeeba930d0706500.camel@infradead.org> <18e854e2a84750c2de2d32384710132b83d84286.camel@infradead.org> <0b9d3901-c10b-effd-6278-6afd1e95b09e@oracle.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-pBonyPERd0LpoyzHKEWs" X-Mailer: Evolution 3.28.5-0ubuntu0.18.04.2 Mime-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by merlin.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-pBonyPERd0LpoyzHKEWs Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2020-11-30 at 15:08 +0000, Joao Martins wrote: > On 11/30/20 12:55 PM, David Woodhouse wrote: > > On Mon, 2020-11-30 at 12:17 +0000, Joao Martins wrote: > > > On 11/30/20 9:41 AM, David Woodhouse wrote: > > > > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote: > > > One thing I didn't quite do at the time, is the whitelisting of unreg= istered > > > ports to userspace. Right now, it's a blacklist i.e. if it's not hand= led in > > > the kernel (IPIs, timer vIRQ, etc) it goes back to userspace. When th= e only > > > ones which go to userspace should be explicitly requested as such > > > and otherwise return -ENOENT in the hypercall. > >=20 > > Hm, why would -ENOENT be a fast path which needs to be handled in the > > kernel? > >=20 >=20 > It's not that it's a fast path. >=20 > Like sending an event channel to an unbound vector, now becomes an possib= le vector to > worry about in userspace VMM e.g. should that port lookup logic be fragil= e. >=20 > So it's more along the lines of Nack-ing the invalid port earlier to rath= er go > to go userspace to invalidate it, provided we do the lookup anyway in the= kernel. If the port lookup logic is fragile, I *want* it in the sandboxed userspace VMM and not in the kernel :) And unless we're going to do *all* of the EVTCHNOP_bind*, EVTCHN_close, etc. handling in the kernel, doesn't userspace have to have all that logic for managing the port space anyway? I think it's better to let userspace own it outright, and use the kernel bypass purely for the fast paths. The VMM can even implement IPI/VIRQ support in userspace, then use the kernel bypass if/when it's available. > > > Perhaps eventfd could be a way to express this? Like if you register > > > without an eventfd it's offloaded, otherwise it's assigned to userspa= ce, > > > or if neither it's then returned an error without bothering the VMM. > >=20 > > I much prefer the simple model where the *only* event channels that the > > kernel knows about are the ones it's expected to handle. > >=20 > > For any others, the bypass doesn't kick in, and userspace gets the > > KVM_EXIT_HYPERCALL exit. > >=20 >=20 > /me nods >=20 > I should comment on your other patch but: if we're going to make it gener= ic for > the userspace hypercall handling, might as well move hyper-v there too. I= n this series, > I added KVM_EXIT_XEN, much like it exists KVM_EXIT_HYPERV -- but with a g= eneric version > I wonder if a capability could gate KVM_EXIT_HYPERCALL to handle both gue= st types, while > disabling KVM_EXIT_HYPERV. But this is probably subject of its own separa= te patch :) There's a limit to how much consolidation we can do because the ABI is different; the args are in different registers. I do suspect Hyper-V should have marshalled its arguments into the existing kvm_run->arch.hypercall and used KVM_EXIT_HYPERCALL but I don't think it makes sense to change it now since it's a user-facing ABI. I don't want to follow its lead by inventing *another* gratuitous exit type for Xen though. --=-pBonyPERd0LpoyzHKEWs Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCECow ggUcMIIEBKADAgECAhEA4rtJSHkq7AnpxKUY8ZlYZjANBgkqhkiG9w0BAQsFADCBlzELMAkGA1UE BhMCR0IxGzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgG A1UEChMRQ09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhl bnRpY2F0aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0EwHhcNMTkwMTAyMDAwMDAwWhcNMjIwMTAxMjM1 OTU5WjAkMSIwIAYJKoZIhvcNAQkBFhNkd213MkBpbmZyYWRlYWQub3JnMIIBIjANBgkqhkiG9w0B AQEFAAOCAQ8AMIIBCgKCAQEAsv3wObLTCbUA7GJqKj9vHGf+Fa+tpkO+ZRVve9EpNsMsfXhvFpb8 RgL8vD+L133wK6csYoDU7zKiAo92FMUWaY1Hy6HqvVr9oevfTV3xhB5rQO1RHJoAfkvhy+wpjo7Q cXuzkOpibq2YurVStHAiGqAOMGMXhcVGqPuGhcVcVzVUjsvEzAV9Po9K2rpZ52FE4rDkpDK1pBK+ 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