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Wed, 8 Jul 2020 21:32:41 +0000 (UTC) Date: Wed, 8 Jul 2020 17:32:40 -0400 From: Eduardo Habkost To: Peter Maydell Subject: Re: [PATCH] cpu: Add starts_halted() method Message-ID: <20200708213240.GC780932@habkost.net> References: <87y2nu3nxq.fsf@morokweng.localdomain> <20200708100038.GG18595@umbus.fritz.box> <20200708152540.GZ7276@habkost.net> <20200708160316.GA7276@habkost.net> <20200708173620.GB780932@habkost.net> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=ehabkost@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=ehabkost@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/07 17:25:10 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Laurent Vivier , Thomas Huth , Alex =?iso-8859-1?Q?Benn=E9e?= , QEMU Developers , qemu-ppc , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Thiago Jung Bauermann , David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Jul 08, 2020 at 09:11:55PM +0100, Peter Maydell wrote: > On Wed, 8 Jul 2020 at 18:36, Eduardo Habkost wrote: > > > > On Wed, Jul 08, 2020 at 06:09:49PM +0100, Peter Maydell wrote: > > > Exactly. It appears that there's a bug in our mechanisms, > > > which is why I'm suggesting that the right thing is > > > to fix that bug rather than marking the CPU as halted > > > earlier in the reset process so that the KVM_RUN happens > > > to do nothing... > > > > I agree this is necessary, but it doesn't seem sufficient. > > > > Having cpu_reset() set halted=0 on spapr (and probably other > > machines) is also a bug, as it could still trigger unwanted > > KVM_RUN when cpu_reset() returns (and before machine code sets > > halted=1). > > The Arm handling of starting-halted sets halted=1 within cpu_reset, > based on whether the CPU object was created with a > "start-powered-off" property. Making this mechanism generic sounds like a good idea. > > I'm not sure in practice that anything can get in asynchronously > and cause a KVM_RUN in between spapr_reset_vcpu() calling > cpu_reset() and it setting cs->halted (and the other stuff), > though. This function ought to be called with the iothread > lock held, so KVM_RUN will only happen if it calls some > other function which incorrectly lets the CPU run. Yeah, maybe it won't happen in practice. It just seems fragile. The same way ppc_cpu_reset() kicked the CPU by accident, code outside cpu_reset() might one day kick the CPU by accident before setting halted=1. -- Eduardo