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=-7.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 6D826C433E0 for ; Wed, 27 Jan 2021 12:45:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1BFD1207B1 for ; Wed, 27 Jan 2021 12:45:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237945AbhA0MpG (ORCPT ); Wed, 27 Jan 2021 07:45:06 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:35752 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237427AbhA0Ml5 (ORCPT ); Wed, 27 Jan 2021 07:41:57 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611751228; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QUziTB7yjFCtXkFHnLPvHhuspDqt9BvMkArF+T1jL44=; b=c6+OHF3eaPYH7lGGo8ZDJn622l3vmB8jLsNIDQCotczYF/jHVQi3ZDuLwUSLwUB4uwgKmj MlH6xwDNbofn4ad6BrVT8xvj3QS39IGQWSOZYD0WzgdhtARtkOdHJ+SGvP/3j+jkO079HX 8rlcqCFSD/aQGNeL85B26vnoJ76Bho4= Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-167-UGS5im4NOrG_BesyskW9OA-1; Wed, 27 Jan 2021 07:40:26 -0500 X-MC-Unique: UGS5im4NOrG_BesyskW9OA-1 Received: by mail-ed1-f69.google.com with SMTP id x13so1284369edi.7 for ; Wed, 27 Jan 2021 04:40:26 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:to:cc:references:from:subject:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=QUziTB7yjFCtXkFHnLPvHhuspDqt9BvMkArF+T1jL44=; b=GvcJqClYLMRiZJWcEv61SFhykwXbYTvmU15DKm/0dkcuL74rDjpYaynbLPii04bxmL cqQjD1TZrRygMTkAl8xxsko0t+Xnte7FaUMusYJ9xM0B46kOX7+T/icxjtItA++NnvH9 /ANqOiYOh4ayuTDOyHn+1fbdSvLFmpYs5NzGm4R4xNRTQvlRW0Aj7hgY5PYIJCtcGGKb U8iM4KVBKLGTgdyUs92hbaKzs0Bxs/wFYltvrO5II+ZJ8O6FQ/1WE1yfrVJgdF9lc7J8 QKGnI+C76XZ4salVFQUj52ib9Wf74PkRIYsV9uhSrGHGC/mSyh+v7WzqPkVyQWJ0vv+Q k6dg== X-Gm-Message-State: AOAM531rJ39vCwNoHPAATtqwBLT0lbO3wwIxo9embfzq0rXzgbb2mF9l BQcwekgbqvOCcTBKBYfR+6+UhjE9pjVXC1/Y2PYFeuZjCxC31W0JFvUR0FGCOW9xvMuexzJ5UXU 1BQi3mnhti2yDZCMsy/wc7FI/ X-Received: by 2002:a17:907:20aa:: with SMTP id pw10mr6561876ejb.314.1611751225514; Wed, 27 Jan 2021 04:40:25 -0800 (PST) X-Google-Smtp-Source: ABdhPJzuit+bGcgdhTrzCjhYmAEkv/7WLRGt5ic9fu4MFMY/cWcP+ETzolxbU4HANZFGkHEb8dAalQ== X-Received: by 2002:a17:907:20aa:: with SMTP id pw10mr6561858ejb.314.1611751225382; Wed, 27 Jan 2021 04:40:25 -0800 (PST) Received: from ?IPv6:2001:b07:6468:f312:c8dd:75d4:99ab:290a? ([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id w18sm767764ejq.59.2021.01.27.04.40.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Jan 2021 04:40:24 -0800 (PST) To: Sean Christopherson Cc: Ben Gardon , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Peter Xu , Peter Shier , Peter Feiner , Junaid Shahid , Jim Mattson , Yulei Zhang , Wanpeng Li , Vitaly Kuznetsov , Xiao Guangrong References: <20210112181041.356734-1-bgardon@google.com> <20210112181041.356734-20-bgardon@google.com> <335d27f7-0849-de37-f380-a5018c5c5535@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock Message-ID: Date: Wed, 27 Jan 2021 13:40:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26/01/21 23:02, Sean Christopherson wrote: >> You can do the deferred freeing with a short write-side critical section to >> ensure all readers have terminated. > > Hmm, the most obvious downside I see is that the zap_collapsible_sptes() case > will not scale as well as the RCU approach. E.g. the lock may be heavily > contested when refaulting all of guest memory to (re)install huge pages after a > failed migration. The simplest solution is to use a write_trylock on the read_unlock() path; if it fails, schedule a delayed work item 1 second in the future so that it's possible to do some batching. (The work item would also have to re-check the llist after each iteration.) Paolo