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 281F221A95D; Sat, 26 Jul 2025 09:19:20 +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=1753521562; cv=none; b=Yh6GPGTjj3dFOd0eWAPejDGspyKE/fd/+d1ZSVaAg7AAJDpcg0S93krNFc2lg0zKhm9Me3WT8UTXEuGTDBfazX78Iz5X5ZyrvqFk2OeIubuvGca/ZnuXiAi6k1rQvqYOrs8v+JYhcXanjzmoORAL9TWaWFs7IoLr/zRi1n9kSyo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753521562; c=relaxed/simple; bh=fR8rSneR4TZPNfJQOykLBbUsidU/r5Uz5YC6zNAHr2o=; h=Date:Message-ID:From:To:Cc:Subject:In-Reply-To:References: MIME-Version:Content-Type; b=buERUnEzGy5dNoQ2ltkpm325DzeHqKc5UcIX8mn/eTBcWvhqgFPZm/y7l5GeRb+iPT08fH3PV6aP6yMGipBHdYMbpiNHlwQzLVHq7OYBkaR/aQjvTq9099hsflGCwpNvbrQXXslpz1Gg05Ebp6yWuaiABo8srDtfUXu5lNCSW5w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Xi3PnjbP; 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="Xi3PnjbP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8F95CC4CEED; Sat, 26 Jul 2025 09:19:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1753521560; bh=fR8rSneR4TZPNfJQOykLBbUsidU/r5Uz5YC6zNAHr2o=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Xi3PnjbPoYkE6hFmmV7xlLdlETPSwi7AYNxBF2RDPKVWFlDtgxxXU+9qiBhK77Lo/ CEyIDpENojBfxRtFhojpzTd7PEVGhSe+y4QLM9395MphdvGY1NiVJ0Afoqn04peF9+ 4vgqIo/gZCM1vuYJP2taYQxL9ptitwEs6KU0kGPOrvOBNs6rl2Wrkee1zR9971lAgN 7ui2M82ISQTXlZcQhRHmQK9CBCbQhcAaYXccAZXj/HsH21iR9awW0HXKXGH6IMu1Hj xD+81P1nCNkUA136liNxanX8rwhUA10gy6ydN+/8h/Ifwruu3NRRGAGMK0HP0t+94u BNRMob1q0Gqzw== Received: from sofa.misterjones.org ([185.219.108.64] helo=lobster-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1ufb3m-001a9m-5j; Sat, 26 Jul 2025 10:19:18 +0100 Date: Sat, 26 Jul 2025 10:19:11 +0100 Message-ID: <87jz3vtils.wl-maz@kernel.org> From: Marc Zyngier To: Wei-Lin Chang Cc: Andre Przywara , Will Deacon , Julien Thierry , kvm@vger.kernel.org, kvmarm@lists.linux.dev, Alexandru Elisei Subject: Re: [PATCH kvmtool v2 5/6] arm64: add FEAT_E2H0 support (TBC) In-Reply-To: References: <20250725144100.2944226-1-andre.przywara@arm.com> <20250725144100.2944226-6-andre.przywara@arm.com> <86cy9o8bwn.wl-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: kvm@vger.kernel.org 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: r09922117@csie.ntu.edu.tw, andre.przywara@arm.com, will@kernel.org, julien.thierry.kdev@gmail.com, kvm@vger.kernel.org, kvmarm@lists.linux.dev, alexandru.elisei@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Sat, 26 Jul 2025 10:01:25 +0100, Wei-Lin Chang wrote: > > Hi all, > > On Fri, Jul 25, 2025 at 05:37:12PM +0100, Marc Zyngier wrote: > > Hi Andre, > > > > Thanks for picking this. A few nits below. > > > > On Fri, 25 Jul 2025 15:40:59 +0100, > > Andre Przywara wrote: > > > > > > From: Marc Zyngier > > > > > > To reduce code complexity, KVM only supports nested virtualisation in > > > VHE mode. So to allow recursive nested virtualisation, and be able to > > > expose FEAT_NV2 to a guest, we must prevent a guest from turning off > > > HCR_EL2.E2H, which is covered by not advertising the FEAT_E2H0 architecture > > > feature. > > > > > > To allow people to run a guest in non-VHE mode, KVM introduced the > > > KVM_ARM_VCPU_HAS_EL2_E2H0 feature flag, which will allow control over > > > HCR_EL2.E2H, but at the cost of turning off FEAT_NV2. > > > > All of that has been captured at length in the kernel code, and I > > think this is "too much information" for userspace. I'd rather we > > stick to a pure description of what the various options mean to the > > user. > > > > > Add a kvmtool command line option "--e2h0" to set that feature bit when > > > creating a guest, to gain non-VHE, but lose recursive nested virt. > > > > How about: > > > > "The --nested option allows a guest to boot at EL2 without FEAT_E2H0 > > (i.e. mandating VHE support). While this is great for "modern" > > operating systems and hypervisors, a few legacy guests are stuck in a > > distant past. > > > > To support those, the --e2h0 option exposes FEAT_E2H0 to the guest, > > at the expense of a number of other features, such as FEAT_NV2. This > > Just a very small thing: > > Will only mentioning FEAT_NV2 here lead people to think that FEAT_NV is > still available with --e2h0? > Maybe s/FEAT_NV2/FEAT_NV/ makes it clearer? Maybe. On the other hand, we never advertise the old FEAT_NV as such, irrespective of the state of E2H. This is indicated by ID_AA64MMFR4_EL1.NV_frac==0b0001 when NV is advertised. So I'm not sure this changes anything, really. Thanks, M. -- Jazz isn't dead. It just smells funny.