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Mon, 25 May 2020 10:17:19 +0100 MIME-Version: 1.0 Date: Mon, 25 May 2020 10:17:18 +0100 From: Marc Zyngier To: Richard Cochran , Jianyong Wu Subject: Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return In-Reply-To: <20200524021106.GC335@localhost> References: <20200522083724.38182-1-jianyong.wu@arm.com> <20200522083724.38182-11-jianyong.wu@arm.com> <20200524021106.GC335@localhost> User-Agent: Roundcube Webmail/1.4.4 Message-ID: <306951e4945b9e486dc98818ba24466d@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: richardcochran@gmail.com, jianyong.wu@arm.com, 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@arm.com, will@kernel.org, suzuki.poulose@arm.com, steven.price@arm.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, Steve.Capper@arm.com, Kaly.Xin@arm.com, justin.he@arm.com, Wei.Chen@arm.com, nd@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Cc: justin.he@arm.com, Wei.Chen@arm.com, kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, sean.j.christopherson@intel.com, steven.price@arm.com, john.stultz@linaro.org, yangbo.lu@nxp.com, pbonzini@redhat.com, tglx@linutronix.de, nd@arm.com, will@kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu 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. 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. For a Linux guest, we always know which reference we're using (the virtual counter). So it is pointless to expose the choice to userspace at all. > >> 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 = 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. 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. M. -- Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm