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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B004FC28B28 for ; Wed, 12 Mar 2025 15:32:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=j0oZAMD7udZnD2znaSjoen/SyP0kj9p+GoRRy/BtAbo=; b=DQvDA1ooZc51StB757J2ANTNJb UmCrcDIbqAByFPrJJE0kr8Qf0U2+/Zfqzu6sLvy4Pu6ZeUJOl89pDIEWEoLmGPIdXOSLkEnXgcevV K8gdjWrtjstDPyLJFKp175HSzO/GRXDyAc7gps6OGYbKMN2s4vBzUWpgrSoTVlZ6ShujOkaS5QME2 zM6kZBTG0HYLqtaVZpILdMUl1OFb0nGyBxyGWt3zPgaHJhZGRwnlaJ6HZp3aNyDtxmOZvytjRztVs rAVGg0cjFB/ZvHGsB9EUirzfZ7e9ZRCBR+f9ua2rXnqLNh/WM7dO12M94hwNuGfItd3edD06hAhC1 zkLgA9BQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1tsO4H-00000008sQP-3ZtW; Wed, 12 Mar 2025 15:32:25 +0000 Received: from dfw.source.kernel.org ([2604:1380:4641:c500::1]) by bombadil.infradead.org with esmtps (Exim 4.98 #2 (Red Hat Linux)) id 1tsO19-00000008s4b-2nKf for linux-arm-kernel@lists.infradead.org; Wed, 12 Mar 2025 15:29:13 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id ED8F55C5BC3; Wed, 12 Mar 2025 15:26:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2151CC4CEDD; Wed, 12 Mar 2025 15:29:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1741793350; bh=tUtskz12THQl2lUAFZuDpLAXb7jPXmXFOy11AYim8Fg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Ki8HNWAykk88TACHZmdKNf4pIY4RMMzd5xWumgtSFOkudpodAzvR2F5wBdz+h0Zz2 nNlRItqijbP4LurgcPamVvWqB4/8nDWGGPJ1xqvRpypPuN3vU03GM4nixfxpDoFz9p Xbzi8UHwR/Q0eWPjitHEFDB/r5WM15MZ1984PcGTnePCSQJ9xlRogHSzYMn+AB75SM Ta58X92WvDaD7O6Os8M5JxFfsEou/oZ6MgQKrXgT9ihtOSr71z6PssS2f6PK14iiho f058iOUTQl74fEt15Jp9mBo/KT0Va2/OLuqFfcqILe9Z4Vuy9DQ3g6kS1fVkb7lA+2 RsXupyrM4PsTg== Date: Wed, 12 Mar 2025 15:29:04 +0000 From: Will Deacon To: Fuad Tabba Cc: Quentin Perret , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, maz@kernel.org, oliver.upton@linux.dev, mark.rutland@arm.com, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, broonie@kernel.org, vdonnefort@google.com Subject: Re: [PATCH v2 3/4] KVM: arm64: Factor out pKVM hyp vcpu creation to separate function Message-ID: <20250312152903.GA6488@willie-the-truck> References: <20250226215520.1759218-1-tabba@google.com> <20250226215520.1759218-4-tabba@google.com> <20250303191811.GA30361@willie-the-truck> <20250303214947.GA30619@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250312_082911_818009_B4C11150 X-CRM114-Status: GOOD ( 42.15 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Mar 04, 2025 at 12:33:27PM +0000, Fuad Tabba wrote: > On Mon, 3 Mar 2025 at 21:49, Will Deacon wrote: > > > > On Mon, Mar 03, 2025 at 07:21:33PM +0000, Fuad Tabba wrote: > > > On Mon, 3 Mar 2025 at 19:18, Will Deacon wrote: > > > > On Mon, Mar 03, 2025 at 07:57:00AM +0000, Fuad Tabba wrote: > > > > > On Fri, 28 Feb 2025 at 19:44, Quentin Perret wrote: > > > > > > On Wednesday 26 Feb 2025 at 21:55:19 (+0000), Fuad Tabba wrote: > > > > > > > static int __pkvm_create_hyp_vm(struct kvm *host_kvm) > > > > > > > { > > > > > > > - size_t pgd_sz, hyp_vm_sz, hyp_vcpu_sz; > > > > > > > + size_t pgd_sz, hyp_vm_sz; > > > > > > > struct kvm_vcpu *host_vcpu; > > > > > > > - pkvm_handle_t handle; > > > > > > > void *pgd, *hyp_vm; > > > > > > > unsigned long idx; > > > > > > > int ret; > > > > > > > @@ -161,33 +178,12 @@ static int __pkvm_create_hyp_vm(struct kvm *host_kvm) > > > > > > > if (ret < 0) > > > > > > > goto free_vm; > > > > > > > > > > > > > > - handle = ret; > > > > > > > + WRITE_ONCE(host_kvm->arch.pkvm.handle, ret); > > > > > > > > > > > > What's the reason to make this a WRITE_ONCE? Does it mean we should > > > > > > update the readers to be READ_ONCE()? > > > > > > > > > > I don't remember the original reason, to be honest. In this case, it > > > > > was to make it consistent with downstream code in Android. That said, > > > > > I plan on revising all of these soon and fixing this (and related > > > > > code) in light of Will's comment regarding potential specter gadgets: > > > > > > > > > > https://lore.kernel.org/all/20250218092705.GA17030@willie-the-truck/ > > > > > > > > I'm not sure the spectre stuff changes the concurrency aspects here, so > > > > Quentin's question presumably still stands even after that. > > > > > > > > Looking at the Android code, this WRITE_ONCE() pairs with a READ_ONCE() > > > > in pkvm_is_hyp_created() which is called without the config_lock held > > > > by kvm_arch_prepare_memory_region(). However, given that > > > > pkvm_is_hyp_created() is only testing against 0, I don't think the > > > > _ONCE() accessors are doing anything useful in that case. > > > > > > > > The more confusing stuff is where 'kvm->arch.pkvm.handle' is read > > > > directly without the 'config_lock' held. The MMU notifiers look like > > > > they can do that, so I wonder if there's a theoretical race where they > > > > can race with first run and issue TLB invalidation with the wrong handle? > > > > That would apply equally to the upstream code, I think. > > > > > > Would using _ONCE accessors with the MMU notifiers be enough to avoid > > > the race, or do we need to reconsider the lock protecting the handle > > > and apply it to the notifiers? > > > > I'm not entirely sure... if we used _ONCE() then we'd get either the > > correct handle or zero, but we presumably need to order that against the > > page-table somehow. The 'mmu_lock' looks like it gives us that, but I > > don't think the notifiers are expecting an uninitialised handle in the > > case where the page-table is empty (i.e. if they fire before first run). > > > > Given that the handle is necessary for TLB invalidation, I'd be inclined > > to make sure that the handle is allocated and published _before_ the > > kvm_pgtable pointer checked by the notifiers is set, but that means > > moving the handle allocation into kvm_arch_init_vm(). Is that do-able? > > It is doable, but it won't be a simple patch. Up until now, the > creation of the hyp view of a vm is associated with the first run of > the vm, rather than its allocation at the host. Part of the reason for > that is that much of the vm state (mainly the vcpu state) isn't > finalized until then. > > This patch series fixes this in part, by decoupling the initialization > of the individual vcpus from the vm. Because of that, it would be > doable, but it might be better as another patch series that we could > test and analyze separately. > > If you agree, I could whip up something and send it soon. Yeah, I think we should fix this as a separate series and drop the WRITE_ONCE() from this patch. Will