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 42268C4361B for ; Tue, 15 Dec 2020 12:46:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 02C83224D4 for ; Tue, 15 Dec 2020 12:46:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728812AbgLOMqG (ORCPT ); Tue, 15 Dec 2020 07:46:06 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54462 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726158AbgLOMqG (ORCPT ); Tue, 15 Dec 2020 07:46:06 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 98343C06179C for ; Tue, 15 Dec 2020 04:45:25 -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=EoTv7mlhkymvP3xeg8QNHwPeTjp+n8e8JQ+yLQUxWdA=; b=RNYUUVBmCZxMXrONddEwPq4kRC YDDNuQLf/G8FhWQ9h4YR6XbjWNQi6+oDFcISVi7RjR8R2IQ5a3IA98LCTFFUd64DNMsZeAuy5j/vh rV9OFwVKScgP6+mwWjom0ZfJAgEpko01bvxMKGOxkB6+hHdQgdVm52aX/9xms3l2o/Dn/62+dmg1L yX8pgShiboYrKVjcD1OD5iIxTyxyQQV8gjnD66GGzbLAHKPwa7GAn4HwE3+RXn52xes3VKL7TaRie DxaQRTcsQnZiBFrfA4SVas67ImhahUzVaCeaXEv+f6BNOHJcFaH83cITX2693IlbIFKLGyBumxYeh O9JG9jCA==; Received: from 54-240-197-234.amazon.com ([54.240.197.234] helo=u3832b3a9db3152.ant.amazon.com) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kp9hi-0006Ky-RX; Tue, 15 Dec 2020 12:45:23 +0000 Message-ID: Subject: Re: [PATCH v3 01/17] KVM: Fix arguments to kvm_{un,}map_gfn() From: David Woodhouse To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: Paolo Bonzini , Ankur Arora , Joao Martins , Boris Ostrovsky , Sean Christopherson , graf@amazon.com, iaslan@amazon.de, pdurrant@amazon.com, aagch@amazon.com, fandree@amazon.com Date: Tue, 15 Dec 2020 12:45:20 +0000 In-Reply-To: <87tusnuvah.fsf@vitty.brq.redhat.com> References: <20201214083905.2017260-1-dwmw2@infradead.org> <20201214083905.2017260-2-dwmw2@infradead.org> <87ft48w0or.fsf@vitty.brq.redhat.com> <6E8FD19B-7ABD-4BF1-84C5-26EDD327F01D@infradead.org> <87a6ugvzek.fsf@vitty.brq.redhat.com> <3E601C94-B52B-43AF-9D13-FD8CB24DED20@infradead.org> <87tusnuvah.fsf@vitty.brq.redhat.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-Ze1XUtUkkJoZVcABrV4G" 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: kvm@vger.kernel.org --=-Ze1XUtUkkJoZVcABrV4G Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2020-12-15 at 13:07 +0100, Vitaly Kuznetsov wrote: > Yes, sadly. Multiple address spaces support was added to KVM as a > generic feature but the only use-case for it at this moment is SMM on > x86 which is 'special', i.e. currently there's only one user for=20 > kvm_map_gfn()/kvm_unmap_gfn() which is steal time accounting and it's > not easy to come up with a use-case when this PV feature needs to be > used from SMM. On the other hand, if we try using multiple address > spaces in KVM for e.g. emulating something like Hyper-V VSM, it becomes > unclear which address space id needs to be used even for steal > time. To be entirely correct, we probably need to remember as_id which > was active when steal time was enabled and stick to it later when we > want to update the data. If we do that, kvm_map_gfn() will lose its only > user. I'm also OK with just completely deleting kvm_map_gfn(). I'm not even using it any more myself; I just kept this patch in my v3 series because it's a sane cleanup. The steal time accounting doesn't *need* to have a kernel mapping of the page just so it can do atomic xchg on it; we can do that just fine on userspace addresses, and do exactly that for futexes. So I'm probably going to knock up a kvm_cmpxchg_guest_offset_cached() and make the steal time code use that, on the *userspace* address. There's already a futex_atomic_cmpxchg_inatomic() which does the actual operation. (It returns -EFAULT for an absent page and lets the caller worry about faulting it in, which isn't quite what I want, but it's a good start and I can either wrap the existing arch-provided function like futexes do, or use it as a basis for a version that waits.) And by an amazing coincidence that's also the function I need for accelerating Xen event channel support... :) --=-Ze1XUtUkkJoZVcABrV4G 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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