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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 558FCC4646D for ; Mon, 6 Aug 2018 11:44:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 10D5821898 for ; Mon, 6 Aug 2018 11:44:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 10D5821898 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730375AbeHFNxi (ORCPT ); Mon, 6 Aug 2018 09:53:38 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:55430 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727109AbeHFNxi (ORCPT ); Mon, 6 Aug 2018 09:53:38 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EDA678011454; Mon, 6 Aug 2018 11:44:54 +0000 (UTC) Received: from [10.36.117.211] (ovpn-117-211.ams2.redhat.com [10.36.117.211]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EEBAC104947E; Mon, 6 Aug 2018 11:44:51 +0000 (UTC) Subject: Re: [PATCH] KVM: try __get_user_pages_fast even if not in atomic context To: Xiao Guangrong , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: rkrcmar@redhat.com, Vitaly Kuznetsov , Junaid Shahid , Xiao Guangrong , Andrea Arcangeli References: <1532706407-11380-1-git-send-email-pbonzini@redhat.com> <54a94492-bb0a-3d2c-6c34-64b6c4336b0a@gmail.com> From: Paolo Bonzini Openpgp: preference=signencrypt Autocrypt: addr=pbonzini@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <54a94492-bb0a-3d2c-6c34-64b6c4336b0a@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Mon, 06 Aug 2018 11:44:55 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Mon, 06 Aug 2018 11:44:55 +0000 (UTC) for IP:'10.11.54.3' DOMAIN:'int-mx03.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'pbonzini@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/08/2018 09:51, Xiao Guangrong wrote: > > > On 07/27/2018 11:46 PM, Paolo Bonzini wrote: >> We are currently cutting hva_to_pfn_fast short if we do not want an >> immediate exit, which is represented by !async && !atomic.  However, >> this is unnecessary, and __get_user_pages_fast is *much* faster >> because the regular get_user_pages takes pmd_lock/pte_lock. >> In fact, when many CPUs take a nested vmexit at the same time >> the contention on those locks is visible, and this patch removes >> about 25% (compared to 4.18) from vmexit.flat on a 16 vCPU >> nested guest. >> > > Nice improvement. > > Then after that, we will unconditionally try hva_to_pfn_fast(), does > it hurt the case that the mappings in the host's page tables have not > been present yet? I don't think so, because that's quite slow anyway. > Can we apply this tech to other places using gup or even squash it > into  get_user_pages()? That may make sense. Andrea, do you have an idea? Paolo