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 D14F7C4332F for ; Wed, 13 Dec 2023 20:05:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1379013AbjLMUFh (ORCPT ); Wed, 13 Dec 2023 15:05:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46964 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229671AbjLMUFe (ORCPT ); Wed, 13 Dec 2023 15:05:34 -0500 Received: from out-185.mta1.migadu.com (out-185.mta1.migadu.com [95.215.58.185]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A496EDC for ; Wed, 13 Dec 2023 12:05:38 -0800 (PST) Date: Wed, 13 Dec 2023 20:05:29 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1702497936; 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=bs3RmWkkBEnHpfKuSLQmU1XdcllYwTl+oGw2E3qh8dc=; b=jJcnDy5bHbUQZ++rOn61pjWjrjxub6x7tQGxN6Q/n2e8x8QqbvQBN0WMjqMDzS9KbV0iYs wCgSpmwEUbVv6YEq3zCdtU1qGuTE6IwHyJzcoTRQw4SP/Qsfm9Sz/cIkb2d4mN4aAQUtts tgUEit1z6/gIrWV69f2FgtcachFAZYI= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Jason Gunthorpe Cc: Catalin Marinas , ankita@nvidia.com, maz@kernel.org, suzuki.poulose@arm.com, yuzenghui@huawei.com, will@kernel.org, alex.williamson@redhat.com, kevin.tian@intel.com, yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org, gshan@redhat.com, linux-mm@kvack.org, lpieralisi@kernel.org, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, mochs@nvidia.com, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for vfio pci devices Message-ID: References: <20231208164709.23101-1-ankita@nvidia.com> <20231208164709.23101-3-ankita@nvidia.com> <20231212181156.GO3014157@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231212181156.GO3014157@nvidia.com> X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Sorry, a bit late to the discussion :) On Tue, Dec 12, 2023 at 02:11:56PM -0400, Jason Gunthorpe wrote: > On Tue, Dec 12, 2023 at 05:46:34PM +0000, Catalin Marinas wrote: > > should know the implications. There's also an expectation that the > > actual driver (KVM guests) or maybe later DPDK can choose the safe > > non-cacheable or write-combine (Linux terminology) attributes for the > > BAR. > > DPDK won't rely on this interface Wait, so what's the expected interface for determining the memory attributes at stage-1? I'm somewhat concerned that we're conflating two things here: 1) KVM needs to know the memory attributes to use at stage-2, which isn't fundamentally different from what's needed for userspace stage-1 mappings. 2) KVM additionally needs a hint that the device / VFIO can handle mismatched aliases w/o the machine exploding. This goes beyond supporting Normal-NC mappings at stage-2 and is really a bug with our current scheme (nGnRnE at stage-1, nGnRE at stage-2). I was hoping that (1) could be some 'common' plumbing for both userspace and KVM mappings. And for (2), any case where a device is intolerant of mismatches && KVM cannot force the memory attributes should be rejected. AFAICT, the only reason PCI devices can get the blanket treatment of Normal-NC at stage-2 is because userspace has a Device-* mapping and can't speculatively load from the alias. This feels a bit hacky, and maybe we should prioritize an interface for mapping a device into a VM w/o a valid userspace mapping. I very much understand that this has been going on for a while, and we need to do *something* to get passthrough working well for devices that like 'WC'. I just want to make sure we don't paint ourselves into a corner that's hard to get out of in the future. -- Thanks, Oliver