linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim Krcmar <rkrcmar@redhat.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Matt Rickard <matt@softrans.com.au>,
	Stephen Boyd <sboyd@kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Florian Weimer <fweimer@redhat.com>,
	KY Srinivasan <kys@microsoft.com>,
	devel@linuxdriverproject.org,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
Date: Thu, 11 Oct 2018 19:27:44 -0300	[thread overview]
Message-ID: <20181011222744.GA17955@amt.cnet> (raw)
In-Reply-To: <CALCETrW6b8=dU6vkXNS-rW1GPzJTbVxuVNsU4aoD_NwwobVQcg@mail.gmail.com>

On Tue, Oct 09, 2018 at 01:09:42PM -0700, Andy Lutomirski wrote:
> On Tue, Oct 9, 2018 at 8:28 AM Marcelo Tosatti <mtosatti@redhat.com> 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 <mtosatti@redhat.com> 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.




  reply	other threads:[~2018-10-11 22:28 UTC|newest]

Thread overview: 81+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-14 12:50 [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Thomas Gleixner
2018-09-14 12:50 ` [patch 01/11] clocksource: Provide clocksource_arch_init() Thomas Gleixner
2018-09-14 12:50 ` [patch 02/11] x86/time: Implement clocksource_arch_init() Thomas Gleixner
2018-09-14 15:45   ` Vitaly Kuznetsov
2018-09-15  6:05     ` Thomas Gleixner
2018-09-14 12:50 ` [patch 03/11] x86/vdso: Enforce 64bit clocksource Thomas Gleixner
2018-09-14 12:50 ` [patch 04/11] x86/vdso: Use unsigned int consistently for vsyscall_gtod_data::seq Thomas Gleixner
2018-09-14 12:50 ` [patch 05/11] x86/vdso: Introduce and use vgtod_ts Thomas Gleixner
2018-09-14 12:50 ` [patch 06/11] x86/vdso: Collapse high resolution functions Thomas Gleixner
2018-09-14 12:50 ` [patch 07/11] x86/vdso: Collapse coarse functions Thomas Gleixner
2018-09-14 12:50 ` [patch 08/11] x86/vdso: Replace the clockid switch case Thomas Gleixner
2018-09-14 12:50 ` [patch 09/11] x86/vdso: Simplify the invalid vclock case Thomas Gleixner
2018-09-17 19:25   ` Andy Lutomirski
2018-09-17 20:12     ` John Stultz
2018-09-18  7:52       ` Thomas Gleixner
2018-09-18  8:30         ` Peter Zijlstra
2018-09-18  8:52           ` Thomas Gleixner
2018-09-18 10:06             ` Thomas Gleixner
2018-09-18 10:41               ` Thomas Gleixner
2018-09-18 12:48                 ` Peter Zijlstra
2018-09-18 13:23                   ` Thomas Gleixner
2018-09-18 13:38                     ` Peter Zijlstra
2018-09-18 15:52                     ` Thomas Gleixner
2018-09-27 14:41                       ` Thomas Gleixner
2018-09-18 14:01         ` Andy Lutomirski
2018-09-18 22:46           ` Thomas Gleixner
2018-09-18 23:03             ` Andy Lutomirski
2018-09-18 23:16               ` Thomas Gleixner
2018-09-27 14:36                 ` Thomas Gleixner
2018-09-27 14:39                   ` Andy Lutomirski
2018-09-19  9:08             ` Rasmus Villemoes
2018-09-19 13:29               ` Thomas Gleixner
2018-09-14 12:50 ` [patch 10/11] x86/vdso: Move cycle_last handling into the caller Thomas Gleixner
2018-09-14 15:26   ` Vitaly Kuznetsov
2018-09-14 12:50 ` [patch 11/11] x66/vdso: Add CLOCK_TAI support Thomas Gleixner
2018-09-14 14:04   ` Andy Lutomirski
2018-09-14 14:27     ` Thomas Gleixner
2018-09-14 14:59       ` Andy Lutomirski
2018-09-16  9:39         ` Thomas Gleixner
2018-09-14 12:56 ` [patch 00/11] x86/vdso: Cleanups, simmplifications and " Florian Weimer
2018-09-14 13:05   ` Thomas Gleixner
2018-09-14 13:06     ` Florian Weimer
2018-09-14 13:19       ` Thomas Gleixner
2018-09-14 13:09   ` Peter Zijlstra
2018-09-14 14:22 ` Arnd Bergmann
2018-09-17 13:00   ` Thomas Gleixner
2018-09-24 21:08     ` Arnd Bergmann
2018-10-03  5:15 ` Andy Lutomirski
2018-10-03  9:22   ` Vitaly Kuznetsov
2018-10-03 10:20     ` Andy Lutomirski
2018-10-03 12:01       ` Vitaly Kuznetsov
2018-10-03 14:20         ` Andy Lutomirski
2018-10-03 15:10           ` Thomas Gleixner
2018-10-03 16:18             ` Andy Lutomirski
2018-10-03 19:06     ` Marcelo Tosatti
2018-10-04  7:54       ` Vitaly Kuznetsov
2018-10-04  8:11         ` Peter Zijlstra
2018-10-04 14:00           ` Andy Lutomirski
2018-10-04 19:31             ` Peter Zijlstra
2018-10-04 20:05               ` Andy Lutomirski
2018-10-04 22:15                 ` Andy Lutomirski
2018-10-06 20:27                   ` Marcelo Tosatti
2018-10-06 22:28                     ` Andy Lutomirski
2018-10-08 15:26                       ` Marcelo Tosatti
2018-10-08 17:38                         ` Andy Lutomirski
2018-10-08 19:36                           ` Marcelo Tosatti
2018-10-09 20:09                             ` Andy Lutomirski
2018-10-11 22:27                               ` Marcelo Tosatti [this message]
2018-10-11 23:00                                 ` Andy Lutomirski
2018-10-15 13:39                                   ` Marcelo Tosatti
2018-10-06 20:49                   ` Marcelo Tosatti
2018-10-04 12:00         ` Paolo Bonzini
2018-10-04 14:04           ` Andy Lutomirski
2018-10-05 21:18         ` Marcelo Tosatti
2018-10-03 19:00   ` Marcelo Tosatti
2018-10-03 19:05     ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support\ Marcelo Tosatti
2018-10-03 22:32     ` [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Andy Lutomirski
2018-10-04 16:37       ` Marcelo Tosatti
2018-10-04 17:08         ` Andy Lutomirski
2018-10-04 17:28           ` Vitaly Kuznetsov
2018-10-04 20:32 ` Andy Lutomirski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20181011222744.GA17955@amt.cnet \
    --to=mtosatti@redhat.com \
    --cc=arnd@arndb.de \
    --cc=devel@linuxdriverproject.org \
    --cc=fweimer@redhat.com \
    --cc=jgross@suse.com \
    --cc=john.stultz@linaro.org \
    --cc=kernellwp@gmail.com \
    --cc=kys@microsoft.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=matt@softrans.com.au \
    --cc=pbonzini@redhat.com \
    --cc=peterz@infradead.org \
    --cc=rkrcmar@redhat.com \
    --cc=sboyd@kernel.org \
    --cc=tglx@linutronix.de \
    --cc=virtualization@lists.linux-foundation.org \
    --cc=vkuznets@redhat.com \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).