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 332CFCCA47A for ; Wed, 8 Jun 2022 20:34:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232003AbiFHUeh (ORCPT ); Wed, 8 Jun 2022 16:34:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52544 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229720AbiFHUeg (ORCPT ); Wed, 8 Jun 2022 16:34:36 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 4D278C16 for ; Wed, 8 Jun 2022 13:34:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654720472; 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=XRSme8wOBaM90PvfwVzl0l/m5nMDDm6ImTO1PuXLlu0=; b=dBEStZglXIDyXjKHasFm2AE1yp5NtRgkbXrUbPn1b5+2rh/jbq8Ko4eKKxF9gDxy+d3sO/ 3RsVGYf1BfSqBGXDbCYuGMQHLVaZ840Cz97h8pzCIUqydxVUk7jLe4SERKupY/OVyEymYJ n6zIiNIPMvaVtLzFt7vvGjf+tJwzM8s= Received: from mail-ej1-f72.google.com (mail-ej1-f72.google.com [209.85.218.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-531-wa0ksZvDPl6ZrL3O9wHv5Q-1; Wed, 08 Jun 2022 16:34:31 -0400 X-MC-Unique: wa0ksZvDPl6ZrL3O9wHv5Q-1 Received: by mail-ej1-f72.google.com with SMTP id fp4-20020a1709069e0400b00711911cecf9so4886539ejc.3 for ; Wed, 08 Jun 2022 13:34:30 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=XRSme8wOBaM90PvfwVzl0l/m5nMDDm6ImTO1PuXLlu0=; b=g9LnrZcfAKAvxiT9RBPbW+7PjuvajiAywIGsx+rtiosghxvmIUxupI72RyMjae/mfE WbL5fLsV1dYOmn5CICe4kUyRK0AfcpDRBCKLBWOmpz8RFSe1V363yxtpuOr75SoGP7xu 2hh9rFR1m96Furn+TgYzs3Kt5oQ8VFkNkNmoJe6ljcB50hDlnu03+k2djdOrMJQ+k9Sb tA5vk/aHe8dAQzeZ06On9i8NIijRkstw/dW6vTxG8oxaqO2QbfARzcqASgGOITK24T+z Dud691HYXMI/BEa+myuqRkSzy6Bj9xp73/t2CujintCE6U0juulG+txZgeCQ2+JHl7Zb Majg== X-Gm-Message-State: AOAM533yOUqpTZ75cpC9GXdUtfesdxMm2gxG6m9uL537sAW3SDoSx76+ GAdlv9+/oqqKr2SoeB545isBcYUvceKYN91IK8CYbj0hnvKEH19uCeF8mcmA3160p+OzbR4NaiJ LpVg/sWZYWOQzyEwrDKPj8A9f6p7RPNGE X-Received: by 2002:a17:906:824a:b0:70f:4c58:6ec6 with SMTP id f10-20020a170906824a00b0070f4c586ec6mr25916967ejx.648.1654720469949; Wed, 08 Jun 2022 13:34:29 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzN/b2zqi1/ofSnqXmKYZ+jg9/ys0C8DxSw3yD646fVpWjItubDhRgDutjwKrhZRWxEWSIEVQnQJwaAKMaz+8Y= X-Received: by 2002:a17:906:824a:b0:70f:4c58:6ec6 with SMTP id f10-20020a170906824a00b0070f4c586ec6mr25916954ejx.648.1654720469668; Wed, 08 Jun 2022 13:34:29 -0700 (PDT) MIME-Version: 1.0 References: <20220301201344.18191-1-sashal@kernel.org> <20220301201344.18191-7-sashal@kernel.org> <5f2b7b93-d4c9-1d59-14df-6e8b2366ca8a@redhat.com> <2d9ba70b-ac18-a461-7a57-22df2c0165c6@redhat.com> <9d336622-6964-454a-605f-1ca90b902836@redhat.com> In-Reply-To: From: Leonardo Bras Soares Passos Date: Wed, 8 Jun 2022 17:34:18 -0300 Message-ID: Subject: Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0 To: Peter Xu Cc: Sean Christopherson , Paolo Bonzini , Sasha Levin , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "Chang S. Bae" , Andy Lutomirski , kvm@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org Hello Peter, On Tue, Jun 7, 2022 at 5:07 PM Peter Xu wrote: > > On Tue, Jun 07, 2022 at 02:17:54PM -0400, Peter Xu wrote: > > On Tue, Jun 07, 2022 at 03:04:27PM +0000, Sean Christopherson wrote: > > > On Tue, Jun 07, 2022, Paolo Bonzini wrote: > > > > On 6/6/22 23:27, Peter Xu wrote: > > > > > On Mon, Jun 06, 2022 at 06:18:12PM +0200, Paolo Bonzini wrote: > > > > > > > However there seems to be something missing at least to me, on why it'll > > > > > > > fail a migration from 5.15 (without this patch) to 5.18 (with this patch). > > > > > > > In my test case, user_xfeatures will be 0x7 (FP|SSE|YMM) if without this > > > > > > > patch, but 0x0 if with it. > > > > > > > > > > > > What CPU model are you using for the VM? > > > > > > > > > > I didn't specify it, assuming it's qemu64 with no extra parameters. > > > > > > > > Ok, so indeed it lacks AVX and this patch can have an effect. > > > > > > > > > > For example, if the source lacks this patch but the destination has it, > > > > > > the source will transmit YMM registers, but the destination will fail to > > > > > > set them if they are not available for the selected CPU model. > > > > > > > > > > > > See the commit message: "As a bonus, it will also fail if userspace tries to > > > > > > set fpu features (with the KVM_SET_XSAVE ioctl) that are not compatible to > > > > > > the guest configuration. Such features will never be returned by > > > > > > KVM_GET_XSAVE or KVM_GET_XSAVE2." > > > > > > > > > > IIUC you meant we should have failed KVM_SET_XSAVE when they're not aligned > > > > > (probably by failing validate_user_xstate_header when checking against the > > > > > user_xfeatures on dest host). But that's probably not my case, because here > > > > > KVM_SET_XSAVE succeeded, it's just that the guest gets a double fault after > > > > > the precopy migration completes (or for postcopy when the switchover is > > > > > done). > > > > > > > > Difficult to say what's happening without seeing at least the guest code > > > > around the double fault (above you said "fail a migration" and I thought > > > > that was a different scenario than the double fault), and possibly which was > > > > the first exception that contributed to the double fault. > > > > > > Regardless of why the guest explodes in the way it does, is someone planning on > > > bisecting this (if necessary?) and sending a backport to v5.15? There's another > > > bug report that is more than likely hitting the same bug. > > > > What's the bisection you mentioned? I actually did a bisection and I also > > checked reverting Leo's change can also fix this issue. Or do you mean > > something else? > > Ah, I forgot to mention on the "stable tree decisions": IIUC it also means > we should apply Leo's patch to all the stable trees if possible, then > migrations between them won't trigger the misterous faults anymore, > including when migrating to the latest Linux versions. > > However there's the delimma that other kernels (any kernel that does not > have Leo's patch) will start to fail migrations to the stable branches that > apply Leo's patch too.. IIUC, you commented before that the migration issue should be solved with a QEMU fix, is that correct? That would mean something like 'QEMU is relying on a kernel bug to work', and should be no blocker for fixing the kernel. If that's the case, I think we should apply the fix to every supported stable branch that have the fpku issue, and in parallel come with a qemu fix for that. What do you think about it? Best regards, Leo > So that's kind of a slight pity. It's just IIUC > the stable trees are more important, because it should have a broader > audience (most Linux distros)? > > > > > > > > > https://lore.kernel.org/all/48353e0d-e771-8a97-21d4-c65ff3bc4192@sentex.net > > > > That is kvm64, and I agree it could be the same problem since both qemu64 > > and kvm64 models do not have any xsave feature bit declared in cpuid 0xd, > > so potentially we could be migrating some fpu states to it even with > > user_xfeatures==0 on dest host. > > > > So today I continued the investigation, and I think what's really missing > > is qemu seems to be ignoring the user_xfeatures check for KVM_SET_XSAVE and > > continues even if it returns -EINVAL. IOW, I'm wondering whether we should > > fail properly and start to check kvm_arch_put_registers() retcode. But > > that'll be a QEMU fix, and it'll at least not causing random faults > > (e.g. double faults) in guest but we should fail the migration gracefully. > > > > Sean: a side note is that I can also easily trigger one WARN_ON_ONCE() in > > your commit 98c25ead5eda5 in kvm_arch_vcpu_ioctl_run(): > > > > WARN_ON_ONCE(kvm_lapic_hv_timer_in_use(vcpu)); > > > > It'll be great if you'd like to check that up. > > > > Thanks, > > > > -- > > Peter Xu > > -- > Peter Xu >