From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 909BB280336 for ; Fri, 23 Jan 2026 12:37:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769171840; cv=none; b=F7e27NxnYik0QE4c1SrbQCOjmXyF+Z+Nsu825V0GfiNaYyIjnDajfrcZ2zmxbyS4AbmT6eb8qUPIrY7Mx9OblnyPKvhUvausyCcTCyYhylg/nxAYZDdBcxyW/ZfDbg4w97JItQuvxedfla9WPvdaVeQ2FTtjLE3aAtyBQ51ynO4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769171840; c=relaxed/simple; bh=mwBkwnK3l8wHlAUHktjFvg0/UeqqT/cK72Oq6z6rBLI=; h=Date:Message-ID:From:To:Cc:Subject:In-Reply-To:References: MIME-Version:Content-Type; b=jGPZS/wnDHift8e4tRksHFAeuYtBoWkugbiduifeIGNiHufLkhwXxAlIHL9AKVFPj6EL74epUBPmE2XK7dD8eNLhzFU5k0QOmJtHtKKNJ6kINrdCZx93VHpFgmy72keU9D7xAR8S0XdivyYnGtRltoBIZaQIya52WZ2bTCappTY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=aerUMdn0; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="aerUMdn0" 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) Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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 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.