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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 798DCC433F5 for ; Fri, 29 Oct 2021 19:26:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 62BB761075 for ; Fri, 29 Oct 2021 19:26:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231442AbhJ2T3T (ORCPT ); Fri, 29 Oct 2021 15:29:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56994 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230381AbhJ2T3O (ORCPT ); Fri, 29 Oct 2021 15:29:14 -0400 Received: from mail-pj1-x102d.google.com (mail-pj1-x102d.google.com [IPv6:2607:f8b0:4864:20::102d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4B1B1C061570 for ; Fri, 29 Oct 2021 12:26:45 -0700 (PDT) Received: by mail-pj1-x102d.google.com with SMTP id iq11so919892pjb.3 for ; Fri, 29 Oct 2021 12:26:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=W0F/ULHmaZ6yiKdcrFiyrI98oNqQ2Wlj0UyZO4mnwig=; b=F47FKuzCq49W4X+gP6en7fn4kYhmztJOYUcBSV26Z8eok7j07w09fLc+8XItP5aMYe Z/RfI//diXfKeNL4X/3Qnb0hzZOXA9nXpn55U5d6hKHllYKNVk7OPOFHUVmO1ItFZoMg azPecgxZus3DPs7bqOdXvPMy77BDMDdrvWES2qgOJp6q5/yA+6f3lw9uuB/EO5lMDoI/ kD/IrulQy3tQRv5PYfqc0kY5Qp0c2ylbex2KFRJocKKo0xsTSWTPTj2i0dXzXxO+Swp8 3rk/odoV1H2LP0hv7GbojKNqT5GU2d13Jyx1MECfsVJy6NkAClfkGVsipB4LvasW47PR skTg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=W0F/ULHmaZ6yiKdcrFiyrI98oNqQ2Wlj0UyZO4mnwig=; b=wOTuMbcmaLmGrGx90pE41q3JSimDjhk9aX5IXTUw90DLkrpc+Ardtf3aXFPyYKOWzq 6I2gl3gnc5WA4tLxrE55Oa0+8OrttD1posn35eLxChIeKARtYznxfNnX3cXU7Y+BnJIc GZhEAakNhv58MLhBWhWSetue2TaZEQLEjk9FDHGA07OWWQt0udmPfOYATPw+orL0Y4qV /RdSwfKX44HwultCMHJY5YBrYbC83U/b3B0pLfKh70YztgZSjpziRV5x0RbDIP37s1sJ w/jYVKUGwuUR49mPP8K8AisEU2SodGEa/73T5Ciw4zS0n95+U2H5xgBOW7KlxJ/M0mb9 RJqQ== X-Gm-Message-State: AOAM530iDA0TGJB2SaNFet1a7zgRw6GyiMrIf+h3ITKS1ilgcb0ziKlY 75j9BueugfqXxrewj1sihM4WtA== X-Google-Smtp-Source: ABdhPJwW8BXMaJw7SFj0kyzLqqWfj6DzD2/RqEruvPjTrn6wlXc1lGIxiYDfcNVKF5ztZtzeCxQu7g== X-Received: by 2002:a17:902:bf02:b0:13f:cfdd:804e with SMTP id bi2-20020a170902bf0200b0013fcfdd804emr11296417plb.1.1635535604654; Fri, 29 Oct 2021 12:26:44 -0700 (PDT) Received: from google.com (157.214.185.35.bc.googleusercontent.com. [35.185.214.157]) by smtp.gmail.com with ESMTPSA id 17sm6670375pgr.10.2021.10.29.12.26.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 29 Oct 2021 12:26:44 -0700 (PDT) Date: Fri, 29 Oct 2021 19:26:40 +0000 From: Sean Christopherson To: Vitaly Kuznetsov Cc: Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, llvm@lists.linux.dev, linux-kernel@vger.kernel.org, Ajay Garg , Paolo Bonzini , Nathan Chancellor , Nick Desaulniers Subject: Re: [PATCH] KVM: x86: Shove vp_bitmap handling down into sparse_set_to_vcpu_mask() Message-ID: References: <20211028213408.2883933-1-seanjc@google.com> <87pmrokn16.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 29, 2021, Sean Christopherson wrote: > On Fri, Oct 29, 2021, Sean Christopherson wrote: > > On Fri, Oct 29, 2021, Vitaly Kuznetsov wrote: > > > > + /* If vp_index == vcpu_idx for all vCPUs, fill vcpu_mask directly. */ > > > > + if (likely(!has_mismatch)) > > > > + bitmap = (u64 *)vcpu_mask; > > > > + > > > > + memset(bitmap, 0, sizeof(vp_bitmap)); > > > > > > ... but in the unlikely case has_mismatch == true 'bitmap' is still > > > uninitialized here, right? How doesn't it crash? > > > > I'm sure it does crash. I'll hack the guest to actually test this. > > Crash confirmed. But I don't feel too bad about my one-line goof because the > existing code botches sparse VP_SET, i.e. _EX flows. The spec requires the guest > to explicit specify the number of QWORDS in the variable header[*], e.g. VP_SET > in this case, but KVM ignores that and does a harebrained calculation to "count" > the number of sparse banks. It does this by counting the number of bits set in > valid_bank_mask, which is comically broken because (a) the whole "sparse" thing > should be a clue that they banks are not packed together, (b) the spec clearly > states that "bank = VPindex / 64", (c) the sparse_bank madness makes this waaaay > more complicated than it needs to be, and (d) the massive sparse_bank allocation > on the stack is completely unnecessary because KVM simply ignores everything that > wouldn't fit in vp_bitmap. > > To reproduce, stuff vp_index in descending order starting from KVM_MAX_VCPUS - 1. > > hv_vcpu->vp_index = KVM_MAX_VCPUS - vcpu->vcpu_idx - 1; > > E.g. with an 8 vCPU guest, KVM will calculate sparse_banks_len=1, read zeros, and > do nothing, hanging the guest because it never sends IPIs. Ugh, I can't read. The example[*] clarifies that the "sparse" VP_SET packs things into BankContents. I don't think I imagined my guest hanging though, so something is awry. Back to debugging... [*] https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/datatypes/hv_vp_set#processor-set-example > So v2 will be completely different because the "fix" for the KASAN issue is to > get rid of sparse_banks entirely. > > [1] https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/hypercall-interface#variable-sized-hypercall-input-headers > [2] https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/datatypes/hv_vp_set#sparse-virtual-processor-set