From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54962) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fWSmy-00050p-Dp for qemu-devel@nongnu.org; Fri, 22 Jun 2018 16:36:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fWSmx-0006En-9t for qemu-devel@nongnu.org; Fri, 22 Jun 2018 16:36:12 -0400 Date: Fri, 22 Jun 2018 16:36:04 -0400 From: Aaron Lindsay Message-ID: <20180622203604.GE12424@codeaurora.org> References: <1523997485-1905-1-git-send-email-alindsay@codeaurora.org> <1523997485-1905-4-git-send-email-alindsay@codeaurora.org> <20180622135045.GD12424@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH v4 03/21] target/arm: Reorganize PMCCNTR accesses List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: qemu-arm , Alistair Francis , Wei Huang , Peter Crosthwaite , QEMU Developers , Michael Spradling , Digant Desai On Jun 22 15:08, Peter Maydell wrote: > On 22 June 2018 at 14:50, Aaron Lindsay wrote: > > On Apr 20 11:17, Peter Maydell wrote: > >> On 17 April 2018 at 21:37, Aaron Lindsay wrote: > >> > pmccntr_read and pmccntr_write contained duplicate code that was already > >> > being handled by pmccntr_sync. Consolidate the duplicated code into two > >> > functions: pmccntr_op_start and pmccntr_op_finish. Add a companion to > >> > c15_ccnt in CPUARMState so that we can simultaneously save both the > >> > architectural register value and the last underlying cycle count - this > >> > ensure time isn't lost and will also allow us to access the 'old' > >> > architectural register value in order to detect overflows in later > >> > patches. > >> > > >> > Signed-off-by: Aaron Lindsay > > >> > - /* If the counter is enabled, this stores the last time the counter > >> > - * was reset. Otherwise it stores the counter value > >> > + /* Stores the architectural value of the counter *the last time it was > >> > + * updated* by pmccntr_op_start. Accesses should always be surrounded > >> > + * by pmccntr_op_start/pmccntr_op_finish to guarantee the latest > >> > + * architecturally-corect value is being read/set. > >> > */ > >> > uint64_t c15_ccnt; > >> > + /* Stores the delta between the architectural value and the underlying > >> > + * cycle count during normal operation. It is used to update c15_ccnt > >> > + * to be the correct architectural value before accesses. During > >> > + * accesses, c15_ccnt_delta contains the underlying count being used > >> > + * for the access, after which it reverts to the delta value in > >> > + * pmccntr_op_finish. > >> > + */ > >> > + uint64_t c15_ccnt_delta; > >> > >> So the key question here is: how does this work for VM migration? > > > > To be honest, I'm not sure I fully understand the things I need to be > > looking out for with VM migration. > > > > My guess, though, is that this current implementation is not sufficient. > > Perhaps there needs to be logic to ensure that c15_ccnt is the current > > architectural value before migration and also to setup c15_ccnt_delta to > > be the delta between that architectural value and the underlying cycle > > count upon inbound migration. Does that sound like an approach which > > would fit well within the rest of the migration framework? > > You need to deal with two different situations: > (1) migration from an older QEMU which doesn't have this patchset > (2) migration from a QEMU with this patchset to one with this patchset > > Either: > (a) all the architectural state can be expressed in our existing > state fields in whatever the previous format was -- in this case > you just need to ensure that cpu_pre_save() and cpu_post_load() > put the state there and unpack it again > (b) we were missing some architectural state and really do need > to transfer more over the wire than we were before -- in this case > you need to add a new subsection to the vmstate which has the fields > that contain that new state, and give the subsection a suitable 'needed' > function to indicate when the subsection should be transferred plus > pre_load and post_load functions that allow us to cope correctly with > the case of the older QEMU that doesn't send the subsection. Okay, thanks! I didn't manage to get to this before v5, but look into it more for v6. -Aaron -- Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.