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=-12.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,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 5E1D1C12002 for ; Mon, 19 Jul 2021 07:19:14 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C067661019 for ; Mon, 19 Jul 2021 07:19:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C067661019 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55322 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m5NYW-0003Fw-Nz for qemu-devel@archiver.kernel.org; Mon, 19 Jul 2021 03:19:12 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41232) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5NXg-0001up-2s for qemu-devel@nongnu.org; Mon, 19 Jul 2021 03:18:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:41923) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5NXc-0001xu-FU for qemu-devel@nongnu.org; Mon, 19 Jul 2021 03:18:18 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626679094; 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=GxRElLD39QGFjHsiONf17WVX3Uz/OSl2NDfk7zqFiKo=; b=QbqlpqpTIjWVrkq8vK4Ummt8d9VGdBDQ3nXBmzHFSU/1GWxplrkyRvt7qjrc5yJebtOxot fmuvHA8Bzh2Dz4RGc34czvSI/wkOQIJfwePlH4/cZayjwtxOu7WouhuHUY/5zjgUToEtIx IzW1kUhayoQNAuceWuC50W2umejyQNE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-453-wvLgtXOiPXytN1kypCCUaw-1; Mon, 19 Jul 2021 03:18:10 -0400 X-MC-Unique: wvLgtXOiPXytN1kypCCUaw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2DDC0CC621; Mon, 19 Jul 2021 07:18:09 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-81.ams2.redhat.com [10.36.112.81]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DCE8B18F0A; Mon, 19 Jul 2021 07:18:08 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 43239113865F; Mon, 19 Jul 2021 09:18:07 +0200 (CEST) From: Markus Armbruster To: David Gibson Subject: Re: spapr_events: Sure we may ignore migrate_add_blocker() failure? References: <87tukvaejt.fsf@dusky.pond.sub.org> Date: Mon, 19 Jul 2021 09:18:07 +0200 In-Reply-To: (David Gibson's message of "Mon, 19 Jul 2021 12:31:06 +1000") Message-ID: <87lf62ydow.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.466, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Aravinda Prasad , Ganesh Goudar , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" David Gibson writes: > On Thu, Jul 15, 2021 at 03:32:06PM +0200, Markus Armbruster wrote: >> Commit 2500fb423a "migration: Include migration support for machine >> check handling" adds this: >> >> ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, &local_err); >> if (ret == -EBUSY) { >> /* >> * We don't want to abort so we let the migration to continue. >> * In a rare case, the machine check handler will run on the target. >> * Though this is not preferable, it is better than aborting >> * the migration or killing the VM. >> */ >> warn_report("Received a fwnmi while migration was in progress"); >> } >> >> migrate_add_blocker() can fail in two ways: >> >> 1. -EBUSY: migration is already in progress >> >> Ignoring this one is clearly intentional. The comment explains why. >> I'm taking it at face value (I'm a spapr ignoramus). > > Right. The argument isn't really about papr particularly, except > insofar as understanding what fwnmi is. fwnmi (FirmWare assisted NMI) > is a reporting mechanism for certain low-level hardware failures > (think memory ECC or cpu level faults, IIRC). If we migrate between > detecting and reporting the error, then the particulars we report will > be mostly meaningless since they relate to hardware we're no longer > running on. Hence the migration blocker. > > However, migrating away from a (non-fatal) fwnmi error is a pretty > reasonable response, so we don't want to actually fail a migration if > its already in progress. > >> Aside: I doubt >> the warning is going to help users. > > You're probably right, but it's not very clear how to do better. It > might possibly help someone in tech support explain why the reported > fwnmi doesn't seem to match the hardware the guest is (now) running > on. Perhaps pointing to the actual problem could help: the FWNMI's information is mostly meaningless. >> 2. -EACCES: we're running with -only-migratable >> >> Why may we ignore -only-migratable here? > > Short answer: because I didn't think about that case. Long answer: > I think we probably shoud ignore it anyway. As above, receiving a > fwnmi doesn't really prevent migration, it just means that if you're > unlucky it can report stale information. Since migrating away from a > possibly-dubious host would be a reasonable response to a non-fatal > fwnmi, I don't think we want to simply prohibit fwnmi entirely with > -only-migratable. I think the comment text and placement could be improved to make clear ignoring this failure is intentional, too. How do you like the following? diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c index a8f2cc6bdc..54d8e856d3 100644 --- a/hw/ppc/spapr_events.c +++ b/hw/ppc/spapr_events.c @@ -911,16 +911,14 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered) } } + /* + * Try to block migration while FWNMI is being handled, so the + * machine check handler runs where the information passed to it + * actually makes sense. This won't actually block migration, + * only delay it slightly. If the attempt fails, carry on. + */ ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, NULL); if (ret == -EBUSY) { - /* - * We don't want to abort so we let the migration to continue. - * In a rare case, the machine check handler will run on the target. - * Though this is not preferable, it is better than aborting - * the migration or killing the VM. It is okay to call - * migrate_del_blocker on a blocker that was not added (which the - * nmi-interlock handler would do when it's called after this). - */ warn_report("Received a fwnmi while migration was in progress"); } >> By the way, we leak @local_err on failure. I'll post a patch, but I'd >> like my question answered first.