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 B36F1C4332F for ; Fri, 19 Nov 2021 09:23:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 904EF61AA3 for ; Fri, 19 Nov 2021 09:23:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234197AbhKSJ0w (ORCPT ); Fri, 19 Nov 2021 04:26:52 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46100 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232838AbhKSJ0w (ORCPT ); Fri, 19 Nov 2021 04:26:52 -0500 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 039DFC061574; Fri, 19 Nov 2021 01:23:50 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=s69dtZTPtWdn4nuXhO5u8qh8vqOiDAUWW2VvLvwoldU=; b=wNvYamDoPaQC8pMtQwoV7gojWQ suDorKgPjTpvmHWDm7SHcr3n74C4FV7ACFp3L0p1jYwLc+3f24tcVugZzVrols+ZR3t/8DsMGIKlf IKzoprKdVpWE8uhGnvCTrRhQcxc/xnV4nmufR1j916s+egIWWDdtPxBVkqJg1rcNsZ5b4YKj5+ARf T4RACuqeGJWqt0LqzR+S6xyS8pR7311g6WBVdOBCnHBwUcuSxiwnn93PNrp9Y73cxDnY60z7/ZvM+ +S1RzC5LJe1B8MmbOzszDpl/ZRGaxUk44TjGIa1hxBmZlxkyAj5IqUwujOISgKuh3i0xSHQ7nVWtp iEsLbg4Q==; Received: from [2001:8b0:10b:1:4a2a:e3ff:fe14:8625] (helo=u3832b3a9db3152.ant.amazon.com) by bombadil.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mo07L-009xIr-Us; Fri, 19 Nov 2021 09:23:36 +0000 Message-ID: <0d8cd126fb15488ae1b523f799fe749f02c4cc8c.camel@infradead.org> Subject: Re: [PATCH v3 08/12] KVM: Propagate vcpu explicitly to mark_page_dirty_in_slot() From: David Woodhouse To: Sean Christopherson Cc: Paolo Bonzini , kvm , Boris Ostrovsky , Joao Martins , "jmattson @ google . com" , "wanpengli @ tencent . com" , "vkuznets @ redhat . com" , "mtosatti @ redhat . com" , "joro @ 8bytes . org" , karahmed@amazon.com, Marc Zyngier , James Morse , Alexandru Elisei , Suzuki K Poulose , Catalin Marinas , Will Deacon , Huacai Chen , Aleksandar Markovic , Michael Ellerman , Benjamin Herrenschmidt , Anup Patel , Christian Borntraeger , kvmarm@lists.cs.columbia.edu, linux-arm-kernel , linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-s390@vger.kernel.org Date: Fri, 19 Nov 2021 09:23:29 +0000 In-Reply-To: References: <20211117174003.297096-1-dwmw2@infradead.org> <20211117174003.297096-9-dwmw2@infradead.org> <85d9fec17f32c3eb9e100e56b91af050.squirrel@twosheds.infradead.org> <4c48546b-eb4a-dff7-cc38-5df54f73f5d4@redhat.com> <20b5952e76c54a3a5dfe5a898e3b835404ac6fb1.camel@infradead.org> <35AEC3FD-B46A-451D-B7D5-4B1BDD5407BD@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-xmDH/MHbX/vObHZ+w1qZ" User-Agent: Evolution 3.36.5-0ubuntu1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org --=-xmDH/MHbX/vObHZ+w1qZ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2021-11-18 at 19:46 +0000, Sean Christopherson wrote: > It is sufficient for the current physical CPU to have an active vCPU, whi= ch is > generally guaranteed in the MMU code because, with a few exceptions, popu= lating > SPTEs is done in vCPU context. >=20 > mmap() will never directly trigger SPTE creation, KVM first requires a vC= PU to > fault on the new address. munmap() is a pure zap flow, i.e. won't create= a > present SPTE and trigger the writeback of the dirty bit. OK, thanks. > That's also why I dislike using kvm_get_running_vcpu(); when it's needed,= there's > a valid vCPU from the caller, but it deliberately gets dropped and indire= ctly > picked back up. Yeah. So as things stand we have a kvm_write_guest() function which takes a 'struct kvm *', as well as a kvm_vcpu_write_guest() function which takes a 'struct kvm_vcpu *'. But it is verboten to *use* the kvm_write_guest() or mark_page_dirty() functions unless you actually *do* have an active vCPU. Do so, and the kernel might just crash; not even a graceful failure mode. That's a fairly awful bear trap that has now caught me *twice*. I'm kind of amused that in all my hairy inline asm and pinning and crap for guest memory access, the thing that's been *broken* is where I just used the *existing* kvm_write_wall_clock() which does the simple kvm_write_guest() thing. I think at the very least perhaps we should do something like this in mark_page_dirty_in_slot(): WARN_ON_ONCE(!kvm_get_running_vcpu() || kvm_get_running_vcpu()->kvm !=3D k= vm); (For illustration only; I'd actually use a local vcpu variable *and* pass that vcpu to kvm_dirty_ring_get()) On propagating the caller's vcpu through and killing off the non-vCPU versions of the functions, I'm torn... because even if we insist on *a* vCPU being passed, it might be *the* vCPU, and that's just setting a more subtle trap (which would have bitten my GPC invalidate code, for example). There are other more complex approaches like adding an extra ring, with spinlocks, for the 'not from a vCPU' cases. 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