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 X-Spam-Level: X-Spam-Status: No, score=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0EC2DC433E0 for ; Mon, 25 May 2020 15:28:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D61BD207CB for ; Mon, 25 May 2020 15:28:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590420505; bh=3X4sQ7kcpAcBdWmFPLh2z6FUnu+whu05DPLM1j+9VQY=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=wFXK7bzaSOqZgmBA5uufptHRSuZgvzBZ08/A7PlF7sLLulYeP/rTCUUsbPrg57j0b fEDfV1bm3cA7kpSQuKL88b6IZd1om73Rnm2qqQhodV/uTmRfcyahiCWQZNsrkGL+b6 ZoqP1fWdYIRC0voGX3iocWxOy0/1QKURsbT4w1Ck= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404167AbgEYP2Z (ORCPT ); Mon, 25 May 2020 11:28:25 -0400 Received: from mail.kernel.org ([198.145.29.99]:51378 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2403999AbgEYP2Y (ORCPT ); Mon, 25 May 2020 11:28:24 -0400 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B44D92071A; Mon, 25 May 2020 15:28:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590420503; bh=3X4sQ7kcpAcBdWmFPLh2z6FUnu+whu05DPLM1j+9VQY=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=0bBuzBY5wC1xcpMo04SVEVYyNO8F3N5GWqP7oDFDTbPCo3EUisqzVks3B99I4/hdY /iEgGaONpIC9cmuYkN8WpvOyZ6fzMILw87y7sn4R0VuCPQebUSBI+bsP0/+gZVHcVq /FsxUauOXD8YIvXLhbH0AjW3jJDp8Sul7gdt/vF0= Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1jdF1Z-00FC3W-Mv; Mon, 25 May 2020 16:28:21 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Mon, 25 May 2020 16:28:21 +0100 From: Marc Zyngier To: Jianyong Wu Cc: Richard Cochran , netdev@vger.kernel.org, yangbo.lu@nxp.com, john.stultz@linaro.org, tglx@linutronix.de, pbonzini@redhat.com, sean.j.christopherson@intel.com, Mark Rutland , will@kernel.org, Suzuki Poulose , Steven Price , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, Steve Capper , Kaly Xin , Justin He , Wei Chen , nd Subject: Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return In-Reply-To: References: <20200522083724.38182-1-jianyong.wu@arm.com> <20200522083724.38182-11-jianyong.wu@arm.com> <20200524021106.GC335@localhost> <306951e4945b9e486dc98818ba24466d@kernel.org> User-Agent: Roundcube Webmail/1.4.4 Message-ID: X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: Jianyong.Wu@arm.com, richardcochran@gmail.com, netdev@vger.kernel.org, yangbo.lu@nxp.com, john.stultz@linaro.org, tglx@linutronix.de, pbonzini@redhat.com, sean.j.christopherson@intel.com, Mark.Rutland@arm.com, will@kernel.org, Suzuki.Poulose@arm.com, Steven.Price@arm.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, Steve.Capper@arm.com, Kaly.Xin@arm.com, Justin.He@arm.com, Wei.Chen@arm.com, nd@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-05-25 15:18, Jianyong Wu wrote: > Hi Marc, > >> -----Original Message----- >> From: Marc Zyngier >> Sent: Monday, May 25, 2020 5:17 PM >> To: Richard Cochran ; Jianyong Wu >> >> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org; >> tglx@linutronix.de; pbonzini@redhat.com; >> sean.j.christopherson@intel.com; >> Mark Rutland ; will@kernel.org; Suzuki Poulose >> ; Steven Price ; linux- >> kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; >> kvmarm@lists.cs.columbia.edu; kvm@vger.kernel.org; Steve Capper >> ; Kaly Xin ; Justin He >> ; Wei Chen ; nd >> Subject: Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user >> choose which counter to return >> >> On 2020-05-24 03:11, Richard Cochran wrote: >> > On Fri, May 22, 2020 at 04:37:23PM +0800, Jianyong Wu wrote: >> >> In general, vm inside will use virtual counter compered with host use >> >> phyical counter. But in some special scenarios, like nested >> >> virtualization, phyical counter maybe used by vm. A interface added >> >> in ptp_kvm driver to offer a mechanism to let user choose which >> >> counter should be return from host. >> > >> > Sounds like you have two time sources, one for normal guest, and one >> > for nested. Why not simply offer the correct one to user space >> > automatically? If that cannot be done, then just offer two PHC >> > devices with descriptive names. >> >> There is no such thing as a distinction between nested or non-nested. >> Both counters are available to the guest at all times, and said guest >> can >> choose whichever it wants to use. So the hypervisor (KVM) has to >> support >> both counters as a reference. >> > It's great that we can decide which counter to return in guest kernel. > So we can abandon these code, including patch 9/11 and 10/11, that > expose the interface to userspace to do the decision. > >> For a Linux guest, we always know which reference we're using (the >> virtual >> counter). So it is pointless to expose the choice to userspace at all. >> > So, we should throw these code of deciding counter type in linux > driver away and just keep the hypercall service of providing both > virtual counter and physical counter in linux to server non-linux > guest. > Am I right? Exactly. We control Linux, and so far nothing is using the physical counter directly. It is only using the virtual counter. On the other side, this is *only* Linux. Other operating systems will need to pick the reference clock that matches their own. If one day we change Linux to use the physical counter, we'll have to do the same thing. M. -- Jazz is not dead. It just smells funny...