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 8563013F6DFF for ; Mon, 30 Jul 2018 08:40:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 31C0520873 for ; Mon, 30 Jul 2018 08:40:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 31C0520873 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 S1726651AbeG3KOY (ORCPT ); Mon, 30 Jul 2018 06:14:24 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:43050 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726528AbeG3KOY (ORCPT ); Mon, 30 Jul 2018 06:14:24 -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 A50C28197008; Mon, 30 Jul 2018 08:40:28 +0000 (UTC) Received: from [10.36.117.175] (ovpn-117-175.ams2.redhat.com [10.36.117.175]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4C7E1112D176; Mon, 30 Jul 2018 08:40:26 +0000 (UTC) Subject: Re: [PATCH] KVM: try __get_user_pages_fast even if not in atomic context To: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: rkrcmar@redhat.com, Vitaly Kuznetsov , Junaid Shahid , Xiao Guangrong References: <1532706407-11380-1-git-send-email-pbonzini@redhat.com> From: David Hildenbrand Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <1532706407-11380-1-git-send-email-pbonzini@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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, 30 Jul 2018 08:40:28 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Mon, 30 Jul 2018 08:40:28 +0000 (UTC) for IP:'10.11.54.3' DOMAIN:'int-mx03.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'david@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27.07.2018 17:46, 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. > > Suggested-by: Andrea Arcangeli > Signed-off-by: Paolo Bonzini > --- > virt/kvm/kvm_main.c | 14 ++++++-------- > 1 file changed, 6 insertions(+), 8 deletions(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 861bb20e8451..0f26ff7ddedb 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -1343,18 +1343,16 @@ static inline int check_user_page_hwpoison(unsigned long addr) > } > > /* > - * The atomic path to get the writable pfn which will be stored in @pfn, > - * true indicates success, otherwise false is returned. > + * The fast path to get the writable pfn which will be stored in @pfn, > + * true indicates success, otherwise false is returned. It's also the > + * only part that runs if we can are in atomic context. > */ > -static bool hva_to_pfn_fast(unsigned long addr, bool atomic, bool *async, > - bool write_fault, bool *writable, kvm_pfn_t *pfn) > +static bool hva_to_pfn_fast(unsigned long addr, bool write_fault, > + bool *writable, kvm_pfn_t *pfn) > { > struct page *page[1]; > int npages; > > - if (!(async || atomic)) > - return false; > - > /* > * Fast pin a writable pfn only if it is a write fault request > * or the caller allows to map a writable pfn for a read fault > @@ -1498,7 +1496,7 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async, > /* we can do it either atomically or asynchronously, not both */ > BUG_ON(atomic && async); > > - if (hva_to_pfn_fast(addr, atomic, async, write_fault, writable, &pfn)) > + if (hva_to_pfn_fast(addr, write_fault, writable, &pfn)) > return pfn; > > if (atomic) > Indeed for the !atomic case this looks perfectly valid. And also !async should be fine as we there is nothing to wait for if the page is already in memory. Reviewed-by: David Hildenbrand -- Thanks, David / dhildenb