From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Date: Thu, 11 Oct 2018 19:27:44 -0300 Message-ID: <20181011222744.GA17955@amt.cnet> References: <20181004193150.GQ19272@hirez.programming.kicks-ass.net> <499807AB-E779-40C3-AA3F-E8C77A7770EC@amacapital.net> <20181006202731.GC7129@amt.cnet> <20181008152650.GB27822@amt.cnet> <20181008193632.GA31729@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Andy Lutomirski Cc: Wanpeng Li , Florian Weimer , Juergen Gross , Arnd Bergmann , Radim Krcmar , Peter Zijlstra , X86 ML , LKML , Linux Virtualization , Stephen Boyd , John Stultz , devel@linuxdriverproject.org, Paolo Bonzini , Thomas Gleixner , Matt Rickard List-Id: virtualization@lists.linuxfoundation.org On Tue, Oct 09, 2018 at 01:09:42PM -0700, Andy Lutomirski wrote: > On Tue, Oct 9, 2018 at 8:28 AM Marcelo Tosatti wrote: > > > > On Mon, Oct 08, 2018 at 10:38:22AM -0700, Andy Lutomirski wrote: > > > On Mon, Oct 8, 2018 at 8:27 AM Marcelo Tosatti wrote: > > > > I read the comment three more times and even dug through the git > > > history. It seems like what you're saying is that, under certain > > > conditions (which arguably would be bugs in the core Linux timing > > > code), > > > > I don't see that as a bug. Its just a side effect of reading two > > different clocks (one is CLOCK_MONOTONIC and the other is TSC), > > and using those two clocks to as a "base + offset". > > > > As the comment explains, if you do that, can't guarantee monotonicity. > > > > > actually calling ktime_get_boot_ns() could be non-monotonic > > > with respect to the kvmclock timing. But get_kvmclock_ns() isn't used > > > for VM timing as such -- it's used for the IOCTL interfaces for > > > updating the time offset. So can you explain how my patch is > > > incorrect? > > > > ktime_get_boot_ns() has frequency correction applied, while > > reading masterclock + TSC offset does not. > > > > So the clock reads differ. > > > > Ah, okay, I finally think I see what's going on. In the kvmclock data > exposed to the guest, tsc_shift and tsc_to_system_mul come from > tgt_tsc_khz, whereas master_kernel_ns and master_cycle_now come from > CLOCK_BOOTTIME. So the kvmclock and kernel clock drift apart at a > rate given by the frequency shift and then suddenly agree again every > time the pvclock data is updated. Yes. > Is there a reason to do it this way? Since pvclock updates which update system_timestamp are expensive (must stop all vcpus), they should be avoided. So only HW TSC counts, and used as offset against vcpu's tsc_timestamp.