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 C38A9D74EDB for ; Fri, 23 Jan 2026 15:17:29 +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=haKTlGMXRzHzTeVN0kjKcN2ofryqhJ3KauOfUGQIMo4=; b=sOCITx2M89h0h2x0VNgExgUcLH NlHOug/6SHaPw0+ijizKGn46aS/6UWiDlHIf0ci4UPkB3f8e4aj+6qXWMmf5N5jbM7gi468ELLBdP TWGr17AeeF8QSz+5eFg0+H/1qLzcpRTVuk4QLo9t39M+ND1Qu/eWprkWp0cyS8JgE/MZ9g/25CBn8 3mnaH4LCsGYEIq5TzPj+dPBjhFqeck6cSqh5Ol8hx/o6mFm5iZfpHrb8Z93vSY9AS3uhHJTXHGrpi C/+OS3fbChjwUejSyG2SNklnZGFYopaJ/AJ3BhF2cduB6RgF+MLnJsV98G6o/OP3etpwlRvjvO37J qpi8uFaQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vjIua-000000095Oc-22kd; Fri, 23 Jan 2026 15:17:24 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vjIuY-000000095O4-1xVH for linux-arm-kernel@lists.infradead.org; Fri, 23 Jan 2026 15:17:23 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 491A31515; Fri, 23 Jan 2026 07:17:14 -0800 (PST) Received: from raptor (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0181E3F632; Fri, 23 Jan 2026 07:17:18 -0800 (PST) Date: Fri, 23 Jan 2026 15:17:15 +0000 From: Alexandru Elisei To: Marc Zyngier 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 Message-ID: References: <20260119105651.255693-1-maz@kernel.org> <86pl70bj42.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <86pl70bj42.wl-maz@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260123_071722_547968_0CBF21B1 X-CRM114-Status: GOOD ( 23.85 ) 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 Hi Marc, On Fri, Jan 23, 2026 at 12:37:17PM +0000, Marc Zyngier wrote: > 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. Got it, thanks for the explanation! Thanks, Alex