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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ECD78EB64DC for ; Thu, 6 Jul 2023 10:29:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230521AbjGFK3P (ORCPT ); Thu, 6 Jul 2023 06:29:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33874 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229514AbjGFK3M (ORCPT ); Thu, 6 Jul 2023 06:29:12 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 748E2123 for ; Thu, 6 Jul 2023 03:28:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1688639307; 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: in-reply-to:in-reply-to:references:references; bh=pWSRtLdme6FJ70IV4C93Q8VXnr5yFBXVr0WOwSgrr+g=; b=erTQvC8h8rS8cXO9U5Nx9olT0qHZXZiqPST8Q/hMo/X3U+iwO/Bo66/qMb1O5lS6TbRb91 JRy0hjwMDI6Smowb8ogvtTEYudH8pdFWEu+jH8MHfmohRpWFZZvTrAY2+EM7Mysfb0H0ae BuQ5YvXYINOpnEf2nwNYuL4KkiNTHW8= 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-194-C_ZbWKdJPQad0nV88K3J4w-1; Thu, 06 Jul 2023 06:28:24 -0400 X-MC-Unique: C_ZbWKdJPQad0nV88K3J4w-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 07D42101A54E; Thu, 6 Jul 2023 10:28:24 +0000 (UTC) Received: from localhost (unknown [10.42.28.237]) by smtp.corp.redhat.com (Postfix) with ESMTP id C75BEF5CE3; Thu, 6 Jul 2023 10:28:23 +0000 (UTC) Date: Thu, 6 Jul 2023 11:28:23 +0100 From: "Richard W.M. Jones" To: Richard Henderson Cc: Peter Zijlstra , Arnd Bergmann , Naresh Kamboju , Anders Roxell , Daniel =?iso-8859-1?Q?D=EDaz?= , Benjamin Copeland , linux-kernel@vger.kernel.org, x86@kernel.org, Paolo Bonzini Subject: Re: qemu-x86_64 booting with 8.0.0 stil see int3: when running LTP tracing testing. Message-ID: <20230706102823.GO7636@redhat.com> References: <2d7595b1-b655-4425-85d3-423801bce644@app.fastmail.com> <20230621160655.GL2053369@hirez.programming.kicks-ass.net> <20230704074620.GA17440@redhat.com> <20230705162830.GC17440@redhat.com> <20230705215008.GD17440@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 06, 2023 at 07:30:50AM +0100, Richard Henderson wrote: > On 7/5/23 22:50, Richard W.M. Jones wrote: > >tb_invalidate_phys_range_fast() *is* called, and we end up calling > > tb_invalidate_phys_page_range__locked -> > > tb_phys_invalidate__locked -> > > do_tb_phys_invalidate > > > >Nevertheless the old TB (containing the call to the int3 helper) is > >still called after the code has been replaced with a NOP. > > > >Of course there are 4 MTTCG threads so maybe another thread is in the > >middle of executing the same TB when it gets invalidated. > > Yes. > > >tb_invalidate_phys_page_range__locked goes to some effort to check if > >the current TB is being invalidated and restart the TB, but as far as > >I can see the test can only work for the current core, and won't > >restart the TB on other cores. > > Yes. > > The assumption with any of these sorts of races is that it is "as > if" the other thread has already passed the location of the write > within that block. But by the time this thread has finished > do_tb_phys_invalidate, any other thread cannot execute the same > block *again*. > > There's a race here, and now that I think about it, there's been mail about it in the past: > > https://lore.kernel.org/qemu-devel/cebad06c-48f2-6dbd-6d7f-3a3cf5aebbe3@linaro.org/ > > We take care of the same race for user-only in translator_access, by > ensuring that each translated page is read-only *before* performing > the read for translation. But for system mode we grab the page > locks *after* the reads. Which means there's a race. > > The email above describes the race pretty clearly, with a new TB > being generated before the write is even complete. > > It'll be non-trivial fixing this, because not only do we need to > grab the lock earlier, there are ordering issues for a TB that spans > two pages, in that one must grab the two locks in the correct order > lest we deadlock. Yes I can see how this is hard to fix. Even if we just lock the page containing the first instruction (which we know) before doing translation, we still have a problem when entering tb_link_page() where we would need to only lock the second page, which might cause ordering issues. How about a new per-page lock, which would be grabbed by do_tb_phys_invalidate() and tb_gen_code(), just on the first instruction? It would mean, I think, that no page can be having TBs invalidated and generated at the same time. Or something like scanning the bytes as they are being translated, generate a secure-ish checksum, then recheck it after translation and discard the TB if the code changed. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW