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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 898C2C433F5 for ; Tue, 21 Sep 2021 07:58:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6674861156 for ; Tue, 21 Sep 2021 07:58:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230437AbhIUIAF (ORCPT ); Tue, 21 Sep 2021 04:00:05 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:33948 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230032AbhIUH75 (ORCPT ); Tue, 21 Sep 2021 03:59:57 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 14B202209C; Tue, 21 Sep 2021 07:58:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1632211109; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=TCRoA5a4c/jtpKOQe0J4E/g6/W+lJjMnAbuFoXReAXI=; b=KPniQxBeYVqXHdwPFzuqyH0Io2uibebIpb0Gs1mmoYACl153OpRpCTaitlPwRQsk/fdpkP poel0PC2wlPdsIWDcVYIgV7FFMxhatiQoAXxWv+N2+c/oLJsrD8/DBX/m2ltid6v1DuPpc 4rdcZ+yZriYWCh4gBPFrm8D7LAg7c5M= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id C75C513B9C; Tue, 21 Sep 2021 07:58:28 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id lXReL6SQSWEnFQAAMHmgww (envelope-from ); Tue, 21 Sep 2021 07:58:28 +0000 Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable() To: Jan Beulich Cc: Boris Ostrovsky , Stefano Stabellini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org References: <20210921070226.32021-1-jgross@suse.com> <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com> From: Juergen Gross Message-ID: <599f4e20-a25d-6cd0-ced5-f2deaf997535@suse.com> Date: Tue, 21 Sep 2021 09:58:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="ITflTTCXoVwbmJMFHEzHN1ZWJTbRSWWbR" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --ITflTTCXoVwbmJMFHEzHN1ZWJTbRSWWbR Content-Type: multipart/mixed; boundary="iFHxcl1T3XjGON5fVwYJ2lTk4ZzYTewBV"; protected-headers="v1" From: Juergen Gross To: Jan Beulich Cc: Boris Ostrovsky , Stefano Stabellini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org Message-ID: <599f4e20-a25d-6cd0-ced5-f2deaf997535@suse.com> Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable() References: <20210921070226.32021-1-jgross@suse.com> <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com> In-Reply-To: <99e6cf1d-ce6a-83e5-2e43-12f6c1a89f3f@suse.com> --iFHxcl1T3XjGON5fVwYJ2lTk4ZzYTewBV Content-Type: multipart/mixed; boundary="------------C1C4B23D13142A37EBB8A439" Content-Language: en-US This is a multi-part message in MIME format. --------------C1C4B23D13142A37EBB8A439 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 21.09.21 09:53, Jan Beulich wrote: > On 21.09.2021 09:02, Juergen Gross wrote: >> --- a/arch/x86/xen/irq.c >> +++ b/arch/x86/xen/irq.c >> @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void) >> { >> struct vcpu_info *vcpu; >> =20 >> - /* >> - * We may be preempted as soon as vcpu->evtchn_upcall_mask is >> - * cleared, so disable preemption to ensure we check for >> - * events on the VCPU we are still running on. >> - */ >> - preempt_disable(); >> - >> vcpu =3D this_cpu_read(xen_vcpu); >> vcpu->evtchn_upcall_mask =3D 0; >> =20 >> - /* Doesn't matter if we get preempted here, because any >> - pending event will get dealt with anyway. */ >> + /* >> + * Now preemption could happen, but this is only possible if an even= t >> + * was handled, so missing an event due to preemption is not >> + * possible at all. >> + * The worst possible case is to be preempted and then check events >> + * pending on the old vcpu, but this is not problematic. >> + */ >=20 > I agree this isn't problematic from a functional perspective, but ... >=20 >> barrier(); /* unmask then check (avoid races) */ >> if (unlikely(vcpu->evtchn_upcall_pending)) >> xen_force_evtchn_callback(); >=20 > ... is a stray call here cheaper than ... >=20 >> - >> - preempt_enable(); >=20 > ... the preempt_{dis,en}able() pair? The question is if a stray call in case of preemption (very unlikely) is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling. I'm quite sure removing the preempt_*() calls will be a net benefit. 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