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 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 949BCC19F2B for ; Mon, 1 Aug 2022 15:54:25 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id C064D91025; Mon, 1 Aug 2022 15:54:19 +0000 (UTC) Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by gabe.freedesktop.org (Postfix) with ESMTPS id BDB7D1127E0 for ; Mon, 1 Aug 2022 15:54:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1659369251; 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=bDrzxcK/hdqF7u8DnUOKQqM/ecXSwkQXnfqic59uxkQ=; b=fvr0vBXom4gOmnQZyFA3o+5Q7lCG2q0AqnabgG2DsUDk+0TDXXWdHSNqK+Zf/7evgp+Xry eJPIsnCRX2JlYu4eK8oYLEUanrDTYtuuikw7kKLfuqFu1vn1ARVVt715s3lOHsDckt6IBv xRp+kxvf8Aa0smWHEAc+KC6mFI9L8/o= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-102-9GoOoRyOOP-3kn0drtBscw-1; Mon, 01 Aug 2022 11:54:06 -0400 X-MC-Unique: 9GoOoRyOOP-3kn0drtBscw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id AF7D4101A586; Mon, 1 Aug 2022 15:54:05 +0000 (UTC) Received: from starship (unknown [10.40.194.242]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7D3AB40E80F4; Mon, 1 Aug 2022 15:53:59 +0000 (UTC) Message-ID: Subject: Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally From: Maxim Levitsky To: Sean Christopherson Date: Mon, 01 Aug 2022 18:53:58 +0300 In-Reply-To: <7c4cf32dca42ab84bdb427a9e4862dbf5509f961.camel@redhat.com> References: <20220427200314.276673-1-mlevitsk@redhat.com> <20220427200314.276673-5-mlevitsk@redhat.com> <5ed0d0e5a88bbee2f95d794dbbeb1ad16789f319.camel@redhat.com> <7c4cf32dca42ab84bdb427a9e4862dbf5509f961.camel@redhat.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Wanpeng Li , kvm@vger.kernel.org, David Airlie , Dave Hansen , dri-devel@lists.freedesktop.org, "H. Peter Anvin" , Brijesh Singh , Joerg Roedel , x86@kernel.org, Ingo Molnar , Zhi Wang , Tom Lendacky , intel-gfx@lists.freedesktop.org, Borislav Petkov , Rodrigo Vivi , Thomas Gleixner , intel-gvt-dev@lists.freedesktop.org, Jim Mattson , Tvrtko Ursulin , linux-kernel@vger.kernel.org, Paolo Bonzini , Vitaly Kuznetsov Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Thu, 2022-07-28 at 10:46 +0300, Maxim Levitsky wrote: > On Mon, 2022-07-25 at 16:08 +0000, Sean Christopherson wrote: > > On Wed, Jul 20, 2022, Maxim Levitsky wrote: > > > On Sun, 2022-05-22 at 13:22 +0300, Maxim Levitsky wrote: > > > > On Thu, 2022-05-19 at 16:37 +0000, Sean Christopherson wrote: > > > > > On Wed, Apr 27, 2022, Maxim Levitsky wrote: > > > > > > @@ -5753,6 +5752,10 @@ int kvm_mmu_init_vm(struct kvm *kvm) > > > Now for nested AVIC, this is what I would like to do: > > > > > > - just like mmu, I prefer to register the write tracking notifier, when the > > > VM is created. > > > > > > - just like mmu, write tracking should only be enabled when nested AVIC is > > > actually used first time, so that write tracking is not always enabled when > > > you just boot a VM with nested avic supported, since the VM might not use > > > nested at all. > > > > > > Thus I either need to use the __kvm_page_track_register_notifier too for AVIC > > > (and thus need to export it) or I need to have a boolean > > > (nested_avic_was_used_once) and register the write tracking notifier only > > > when false and do it not on VM creation but on first attempt to use nested > > > AVIC. > > > > > > Do you think this is worth it? I mean there is some value of registering the > > > notifier only when needed (this way it is not called for nothing) but it does > > > complicate things a bit. > > > > Compared to everything else that you're doing in the nested AVIC code, refcounting > > the shared kvm_page_track_notifier_node object is a trivial amount of complexity. > Makes sense. > > > And on that topic, do you have performance numbers to justify using a single > > shared node? E.g. if every table instance has its own notifier, then no additional > > refcounting is needed. > > The thing is that KVM goes over the list of notifiers and calls them for every write from the emulator > in fact even just for mmio write, and when you enable write tracking on a page, > you just write protect the page and add a mark in the page track array, which is roughly > > 'don't install spte, don't install mmio spte, but just emulate the page fault if it hits this page' > > So adding more than a bare minimum to this list, seems just a bit wrong. > > > > It's not obvious that a shared node will provide better > > performance, e.g. if there are only a handful of AVIC tables being shadowed, then > > a linear walk of all nodes is likely fast enough, and doesn't bring the risk of > > a write potentially being stalled due to having to acquire a VM-scoped mutex. > > The thing is that if I register multiple notifiers, they all will be called anyway, > but yes I can use container_of, and discover which table the notifier belongs to, > instead of having a hash table where I lookup the GFN of the fault. > > The above means practically that all the shadow physid tables will be in a linear > list of notifiers, so I could indeed avoid per vm mutex on the write tracking, > however for simplicity I probably will still need it because I do modify the page, > and having per physid table mutex complicates things. > > Currently in my code the locking is very simple and somewhat dumb, but the performance > is very good because the code isn't executed often, most of the time the AVIC hardware > works alone without any VM exits. > > Once the code is accepted upstream, it's one of the things that can be improved. > > > Note though that I still need a hash table and a mutex because on each VM entry, > the guest can use a different physid table, so I need to lookup it, and create it, > if not found, which would require read/write of the hash table and thus a mutex. > > > > > > I can also stash this boolean (like 'bool registered;') into the 'struct > > > kvm_page_track_notifier_node', and thus allow the > > > kvm_page_track_register_notifier to be called more that once - then I can > > > also get rid of __kvm_page_track_register_notifier. > > > > No, allowing redundant registration without proper refcounting leads to pain, > > e.g. X registers, Y registers, X unregisters, kaboom. > > > > True, but then what about adding a refcount to 'struct kvm_page_track_notifier_node' > instead of a boolean, and allowing redundant registration? > Probably not worth it, in which case I am OK to add a refcount to my avic code. > > Or maybe just scrap the whole thing and just leave registration and activation of the > write tracking as two separate things? Honestly now that looks like the most clean > solution. Kind ping on this. Do you still want me to enable write tracking on the notifier registeration, or scrap the idea? Best regards, Maxim Levitsky > > Best regards, > Maxim Levitsky