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 7FA6DD6CFD1 for ; Fri, 23 Jan 2026 12:37:31 +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:Content-Type:MIME-Version: References:In-Reply-To:Subject:Cc:To:From:Message-ID: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=HiQUndjs+Yq9RTVXBjmXjjomA1LGgPnYDnZTra2CTSk=; b=ZaKnxFSCRzQkQ/b8cmW1+Uh7O+ WjFTc6xL65imctfXlBzhbBCXy+XlgJYzOZRHI1Kp6PM3R4lE/UosjF1ioXS6aSYhd1Vc5G27oz2Tk v5KRYketHfITIab3O9a7jTalM1Ig2URv8QUbHZryaqAsQAkY7qX8KbCP1vlT8moiAeAGyUiNq8nVs oj+n5mKojfJVSpi/0NK1vLD9uzUA8s9yvi2FwpK9BeEjcnBQ88OnrXXyJQnpYYlM443i9bUE76mc7 /97M7HJA7w9tsE8eWohCDtAkrPai/luBw1dFMD1zQsQuyZrhgzN5ImRFPEHbVQW3YpPAK40IKKYuZ w9g4HzCg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vjGPj-00000008vBz-2raT; Fri, 23 Jan 2026 12:37:23 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vjGPh-00000008vBI-062h for linux-arm-kernel@lists.infradead.org; Fri, 23 Jan 2026 12:37:22 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 4424743968; Fri, 23 Jan 2026 12:37:20 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 188DBC19422; Fri, 23 Jan 2026 12:37:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769171840; bh=mwBkwnK3l8wHlAUHktjFvg0/UeqqT/cK72Oq6z6rBLI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=aerUMdn0X+/4fq/Ac4kO/JhZirNWx3pRgCiC+vcgAOKYqgbiKqMjsH6eKheOKKc3h /GXq0T9xmKyOrNBosp2vdNAnWhd0NfH1inJBRnebszJk0m1EGqenOsU4ne6sM4syAh EmNJBTfZc+WyzfyZqGZQbN8QZhVIuINjLHZYM5IxasLSFVen9/C2N0hVOO2O3PwoQD tOArUY2DnSkxre4oJQAe9yuZlfCQ+3J564x+JRgy+c6hQTxglbHmIfUism0f5i62CH Zb70vxnAyIyZbpeRk0K+Yci4LqzCHh//12q93KVNUOZm0/LsTe31oUuACbVcmz/07f 73OoMyRb0lDYA== Received: from sofa.misterjones.org ([185.219.108.64] helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.98.2) (envelope-from ) id 1vjGPd-000000053EJ-2l4c; Fri, 23 Jan 2026 12:37:17 +0000 Date: Fri, 23 Jan 2026 12:37:17 +0000 Message-ID: <86pl70bj42.wl-maz@kernel.org> From: Marc Zyngier To: Alexandru Elisei Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, Joey Gouly , Suzuki K Poulose , Oliver Upton , Zenghui Yu , Will Deacon , Quentin Perret , Fuad Tabba Subject: Re: [PATCH 0/6] KVM: arm64: ... and FWB for all In-Reply-To: References: <20260119105651.255693-1-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: alexandru.elisei@arm.com, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, joey.gouly@arm.com, suzuki.poulose@arm.com, oupton@kernel.org, yuzenghui@huawei.com, will@kernel.org, qperret@google.com, tabba@google.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260123_043721_097325_B80E4A20 X-CRM114-Status: GOOD ( 28.92 ) 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 Fri, 23 Jan 2026 12:22:58 +0000, Alexandru Elisei wrote: > > Hi Marc, > > On Mon, Jan 19, 2026 at 10:56:45AM +0000, Marc Zyngier wrote: > > [Yes, you can tell what I was listening to] > > > > Today, running in protected mode puts the host under it's own S2 with > > FWB=0, even if the rest of the guests are running with FWB=1. The > > rationale for this special-casing of the host is that we want the > > combined attributes to actually reflect the ones exposed by S1. > > > > We therefore use NormalCacheable (which is the weakest memory type) as > > the default attributes at S2 so that S1 can only strengthen the final > > memory type. > > > > But there is no reason why we cannot achieve the same effect with > > FWB. We normally use FWB to enforce cacheable memory from S2, > > irrespective of S1. But it is possible to configure the S2 attributes > > as "pass-through", so that the S1 attributes are always used. > > Would you mind clarifying why FWB is preferable? Is it so that pkvm uses > the same approach for configuring stage 2 for the host as for a guest? Or > is it something else? Having only one attribute encoding scheme to think about on a given machine is vastly preferable, specially when you are debugging. Additionally, FWB==0 may not be with us forever. > > This small series implements that change, adding a flag that actually > > describes what we are trying to do (instead of the NOFWB flag which is > > pretty obscure), and fixes an interesting gotcha with CMOs. > > Interestingly, CMOs never actually worked with NOFWB on an FWB-aware > > platform... > > Hmm... as far I can tell pkvm doesn't populate > mm_ops->dcache_clean_inval_poc for host_mmu, so pKVM doesn't issue CMOs for > the host s2, with or without these changes. Yeah, I clearly was talking nonsense. I don't know how I came to this silly conclusion. Thanks, M. -- Without deviation from the norm, progress is not possible.