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Mon, 25 May 2020 14:18:19 +0000 From: Jianyong Wu To: Marc Zyngier , Richard Cochran 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 Thread-Topic: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return Thread-Index: AQHWMBRtHtaBVLC+ukSXkuIjkrpVgai2gQEAgAIJaQCAAEKwQA== Date: Mon, 25 May 2020 14:18:18 +0000 Message-ID: References: <20200522083724.38182-1-jianyong.wu@arm.com> <20200522083724.38182-11-jianyong.wu@arm.com> <20200524021106.GC335@localhost> <306951e4945b9e486dc98818ba24466d@kernel.org> In-Reply-To: <306951e4945b9e486dc98818ba24466d@kernel.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ts-tracking-id: f812874b-8ee4-45b7-9487-bdd76b3898df.1 x-checkrecipientchecked: true Authentication-Results-Original: kernel.org; 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X-MS-Office365-Filtering-Correlation-Id-Prvs: cf8685cc-81c1-4ff3-3952-08d800b6793b X-Forefront-PRVS: 0414DF926F X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: IHldoTuD5axLuvU+YXBgRJ7IHfaCH114FFTOi6CBV74g6MHKl6TzViC/EZBgrTZ5Wt8YUZ6m//oyczz+XLEOaff9g/LMqzquQnhOteVJvFKGGKyxpij7Q2oXZo2RswjewaeRecpUFgE55FVrdroXFGpQP8XTXXOeFY+KgoOaPEX6RMFfHWGfBtmdzfeP3o1k77YWtwC53Q3Dfz7BiUJ+596YrnjkdwKNhDq8d9+xC+5U9OFhux+PJ7ev8yAAwdh3pXq8fXrZmCZkoeV6IgqvtQCikwz1jP2UeUFl7vxSPOGlH1c6zG6Zl23gfPHOVAqFh3znYGy+RZPucWNx1rYhjrqVWerZcKZpxL8df1y1OWzEqc0dWaM7gP1w1+DVym3hCMk8aVxMVlif9JwKvyDzOw== X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 May 2020 14:18:28.9440 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: bbed0503-8ac8-4e91-8b63-08d800b67f1c X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: VI1PR08MB4093 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 >=20 > 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. >=20 > 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. >=20 It's great that we can decide which counter to return in guest kernel. So w= e can abandon these code, including patch 9/11 and 10/11, that expose the i= nterface to userspace to do the decision. > For a Linux guest, we always know which reference we're using (the virtua= l > counter). So it is pointless to expose the choice to userspace at all. >=20 So, we should throw these code of deciding counter type in linux driver awa= y and just keep the hypercall service of providing both virtual counter and= physical counter in linux to server non-linux guest. Am I right? > > > >> diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c > >> index fef72f29f3c8..8b0a7b328bcd 100644 > >> --- a/drivers/ptp/ptp_chardev.c > >> +++ b/drivers/ptp/ptp_chardev.c > >> @@ -123,6 +123,9 @@ long ptp_ioctl(struct posix_clock *pc, unsigned > >> int cmd, unsigned long arg) > >> struct timespec64 ts; > >> int enable, err =3D 0; > >> > >> +#ifdef CONFIG_ARM64 > >> + static long flag; > > > > static? This is not going to fly. > > > >> + * In most cases, we just need virtual counter from host and > >> + * there is limited scenario using this to get physical counter > >> + * in guest. > >> + * Be careful to use this as there is no way to set it back > >> + * unless you reinstall the module. > > > > How on earth is the user supposed to know this? > > > > From your description, this "flag" really should be a module > > parameter. >=20 > Not even that. If anything, the driver can obtain full knowledge of which > counter is in use without any help. And the hard truth is that it is > *always* the virtual counter as far as Linux is concerned. Good! Thanks Jianyong=20 =20 >=20 > M. > -- > Jazz is not dead. It just smells funny...