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=-4.0 required=3.0 tests=BAYES_00,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 7839BC433DB for ; Wed, 27 Jan 2021 17:24:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2994E64DA5 for ; Wed, 27 Jan 2021 17:24:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233678AbhA0RYo (ORCPT ); Wed, 27 Jan 2021 12:24:44 -0500 Received: from mail.kernel.org ([198.145.29.99]:41400 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232218AbhA0RYf (ORCPT ); Wed, 27 Jan 2021 12:24:35 -0500 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 62CD064DA0; Wed, 27 Jan 2021 17:23:53 +0000 (UTC) Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94) (envelope-from ) id 1l4oXn-00AQmI-8F; Wed, 27 Jan 2021 17:23:51 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 27 Jan 2021 17:23:50 +0000 From: Marc Zyngier To: Alexandru Elisei Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, James Morse , Julien Thierry , Suzuki K Poulose , Eric Auger , kernel-team@android.com Subject: Re: [PATCH v2 6/7] KVM: arm64: Upgrade PMU support to ARMv8.4 In-Reply-To: References: <20210125122638.2947058-1-maz@kernel.org> <20210125122638.2947058-7-maz@kernel.org> <59700102-5340-b5ec-28e2-d95ee3e59c6b@arm.com> <1b594e7b1f47e372ea84f759507db0b9@kernel.org> User-Agent: Roundcube Webmail/1.4.10 Message-ID: <54926337c1779a492f94f57ee63e5c69@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: alexandru.elisei@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, eric.auger@redhat.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 2021-01-27 17:00, Alexandru Elisei wrote: > Hi Marc, > > On 1/27/21 2:35 PM, Marc Zyngier wrote: >> Hi Alex, >> >> On 2021-01-27 14:09, Alexandru Elisei wrote: >>> Hi Marc, >>> >>> On 1/25/21 12:26 PM, Marc Zyngier wrote: >>>> Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be >>>> pretty easy. All that is required is support for PMMIR_EL1, which >>>> is read-only, and for which returning 0 is a valid option as long >>>> as we don't advertise STALL_SLOT as an implemented event. >>> >>> According to ARM DDI 0487F.b, page D7-2743: >>> >>> "If ARMv8.4-PMU is implemented: >>> - If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED >>> whether the PMMIR >>> System registers are implemented. >>> - If STALL_SLOT is implemented, then the PMMIR System registers are >>> implemented." >>> >>> I tried to come up with a reason why PMMIR is emulated instead of >>> being left >>> undefined, but I couldn't figure it out. Would you mind adding a >>> comment or >>> changing the commit message to explain that? >> >> The main reason is that PMMIR gets new fields down the line, >> and doing the bare minimum in term of implementation allows >> us to gently ease into it. > I think I understand what you are saying - add a bare minimum emulation > of the > PMMIR register now so it's less work when we do decide to support the > STALL_SLOT > event for a guest. >> >> We could also go for the full PMMIR reporting on homogeneous >> systems too, as a further improvement. >> >> What do you think? > > I don't have an opinion either way. But if you do decide to add full > emulation for > STALL_SLOT, I would like to help with reviewing the patches (I'm > curious to see > how KVM will detect that it's running on a homogeneous system). I'm not sure we can *detect* it. We'd need some more information from the core arch code and firmware. To be honest, PMU emulation is a joke on BL, so maybe we shouldn't really care and expose what we have. M. -- Jazz is not dead. It just smells funny...