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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE6FDC433EF for ; Thu, 13 Jan 2022 13:01:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234961AbiAMNBT (ORCPT ); Thu, 13 Jan 2022 08:01:19 -0500 Received: from foss.arm.com ([217.140.110.172]:44622 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234951AbiAMNBR (ORCPT ); Thu, 13 Jan 2022 08:01:17 -0500 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 0ECC9ED1; Thu, 13 Jan 2022 05:01:17 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.5.145]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5FC7F3F766; Thu, 13 Jan 2022 05:01:11 -0800 (PST) Date: Thu, 13 Jan 2022 13:01:08 +0000 From: Mark Rutland To: Marc Zyngier Cc: linux-kernel@vger.kernel.org, aleksandar.qemu.devel@gmail.com, alexandru.elisei@arm.com, anup.patel@wdc.com, aou@eecs.berkeley.edu, atish.patra@wdc.com, benh@kernel.crashing.org, borntraeger@linux.ibm.com, bp@alien8.de, catalin.marinas@arm.com, chenhuacai@kernel.org, dave.hansen@linux.intel.com, david@redhat.com, frankja@linux.ibm.com, frederic@kernel.org, gor@linux.ibm.com, hca@linux.ibm.com, imbrenda@linux.ibm.com, james.morse@arm.com, jmattson@google.com, joro@8bytes.org, kvm@vger.kernel.org, mingo@redhat.com, mpe@ellerman.id.au, nsaenzju@redhat.com, palmer@dabbelt.com, paulmck@kernel.org, paulus@samba.org, paul.walmsley@sifive.com, pbonzini@redhat.com, seanjc@google.com, suzuki.poulose@arm.com, tglx@linutronix.de, tsbogend@alpha.franken.de, vkuznets@redhat.com, wanpengli@tencent.com, will@kernel.org Subject: Re: [PATCH 1/5] kvm: add exit_to_guest_mode() and enter_from_guest_mode() Message-ID: References: <20220111153539.2532246-1-mark.rutland@arm.com> <20220111153539.2532246-2-mark.rutland@arm.com> <87v8yqrwcs.wl-maz@kernel.org> <87pmov97fk.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pmov97fk.wl-maz@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 13, 2022 at 11:55:11AM +0000, Marc Zyngier wrote: > On Thu, 13 Jan 2022 11:01:30 +0000, > Mark Rutland wrote: > > > > On Tue, Jan 11, 2022 at 05:54:59PM +0000, Marc Zyngier wrote: > > > Hi Mark, > > > > > > On Tue, 11 Jan 2022 15:35:35 +0000, > > > Mark Rutland wrote: > > [...] > > > > > +/* > > > > + * Enter guest context and enter an RCU extended quiescent state. > > > > + * > > > > + * This should be the last thing called before entering the guest, and must be > > > > + * called after any potential use of RCU (including any potentially > > > > + * instrumented code). > > > > > > nit: "the last thing called" is terribly ambiguous. Any architecture > > > obviously calls a ****load of stuff after this point. Should this be > > > 'the last thing involving RCU' instead? > > > > I agree this is unclear and I struggled to fing good wording for this. Is the > > following any better? > > > > /* > > * Enter guest context and enter an RCU extended quiescent state. > > * > > * Between guest_context_enter_irqoff() and guest_context_exit_irqoff() it is > > * unsafe to use any code which may directly or indirectly use RCU, tracing > > * (including IRQ flag tracing), or lockdep. All code in this period must be > > * non-instrumentable. > > */ > > > > If that's good I can add similar to guest_context_exit_irqoff(). > > Yes, that's much clearer, thanks. > > > > > [...] > > > > > > +/** > > > > + * exit_to_guest_mode - Fixup state when exiting to guest mode > > > > + * > > > > + * This is analagous to exit_to_user_mode(), and ensures we perform the > > > > + * following in order: > > > > + * > > > > + * 1) Trace interrupts on state > > > > + * 2) Invoke context tracking if enabled to adjust RCU state > > > > + * 3) Tell lockdep that interrupts are enabled > > > > > > nit: or rather, are about to be enabled? Certainly on arm64, the > > > enable happens much later, right at the point where we enter the guest > > > for real. > > > > True; I'd cribbed the wording from the comment block above exit_to_user_mode(), > > but I stripped the context that made that clear. I'll make that: > > > > /** > > * exit_to_guest_mode - Fixup state when exiting to guest mode > > * > > * Entry to a guest will enable interrupts, but the kernel state is > > * interrupts disabled when this is invoked. Also tell RCU about it. > > * > > * 1) Trace interrupts on state > > * 2) Invoke context tracking if enabled to adjust RCU state > > * 3) Tell lockdep that interrupts are enabled > > * > > * Invoked from architecture specific code before entering a guest. > > * Must be called with interrupts disabled and the caller must be > > * non-instrumentable. > > * The caller has to invoke guest_timing_enter_irqoff() before this. > > * > > * Note: this is analagous to exit_to_user_mode(). > > nit: analogous > > > */ > > > > ... with likewise for enter_from_guest_mode(), if that's clear enough? > > Yes, that's great. Thanks; I've pushed out an updated branch with those changes (including the typo fixes). I'll wait until next week before sending out a v2 since I don't think that meaningfully affects the arch bits for other architectures. Mark.