From owner-svn-src-head@freebsd.org Sun Oct 29 20:54:58 2017 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id A9EF1E4C4A6 for ; Sun, 29 Oct 2017 20:54:58 +0000 (UTC) (envelope-from ian@freebsd.org) Received: from pmta2.delivery6.ore.mailhop.org (pmta2.delivery6.ore.mailhop.org [54.200.129.228]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 896316692B for ; Sun, 29 Oct 2017 20:54:58 +0000 (UTC) (envelope-from ian@freebsd.org) X-MHO-User: 4b28e1d1-bceb-11e7-b50b-53dc5ecda239 X-Report-Abuse-To: https://support.duocircle.com/support/solutions/articles/5000540958-duocircle-standard-smtp-abuse-information X-Originating-IP: 73.78.92.27 X-Mail-Handler: DuoCircle Outbound SMTP Received: from ilsoft.org (unknown [73.78.92.27]) by outbound2.ore.mailhop.org (Halon) with ESMTPSA id 4b28e1d1-bceb-11e7-b50b-53dc5ecda239; Sun, 29 Oct 2017 20:54:02 +0000 (UTC) Received: from rev (rev [172.22.42.240]) by ilsoft.org (8.15.2/8.15.2) with ESMTP id v9TKstof002004; Sun, 29 Oct 2017 14:54:55 -0600 (MDT) (envelope-from ian@freebsd.org) Message-ID: <1509310495.21609.60.camel@freebsd.org> Subject: Re: svn commit: r325108 - head/sys/amd64/vmm/io From: Ian Lepore To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Date: Sun, 29 Oct 2017 14:54:55 -0600 In-Reply-To: <201710292050.v9TKo3j5058456@repo.freebsd.org> References: <201710292050.v9TKo3j5058456@repo.freebsd.org> Content-Type: text/plain; charset="ISO-8859-1" X-Mailer: Evolution 3.18.5.1 FreeBSD GNOME Team Port Mime-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 29 Oct 2017 20:54:58 -0000 On Sun, 2017-10-29 at 20:50 +0000, Ian Lepore wrote: > Author: ian > Date: Sun Oct 29 20:50:03 2017 > New Revision: 325108 > URL: https://svnweb.freebsd.org/changeset/base/325108 > > Log: >   Improve the performance of the hpet timer in bhyve guests by making the >   timer frequency a power of two.  This changes the frequency from 10 to >   16.7 MHz (2 ^ 24 HZ).  Using a power of two avoids roundoff errors when >   doing arithmetic in sbintime_t units. >    >   Testing shows this can fix erratic ntpd behavior in guests using the >   hpet timer (which is the default for multicore guests). >    >   Reported by: bsam@ > > Modified: >   head/sys/amd64/vmm/io/vhpet.c > > Modified: head/sys/amd64/vmm/io/vhpet.c > ============================================================================== > --- head/sys/amd64/vmm/io/vhpet.c Sun Oct 29 20:40:56 2017 (r325107) > +++ head/sys/amd64/vmm/io/vhpet.c Sun Oct 29 20:50:03 2017 (r325108) > @@ -51,7 +51,7 @@ __FBSDID("$FreeBSD$"); >   >  static MALLOC_DEFINE(M_VHPET, "vhpet", "bhyve virtual hpet"); >   > -#define HPET_FREQ 10000000 /* 10.0 Mhz */ > +#define HPET_FREQ 16777216 /* 16.7 (2^24) Mhz */ >  #define FS_PER_S 1000000000000000ul >   >  /* Timer N Configuration and Capabilities Register */ > It should be noted that this is really more of a workaround than a fix.  I think the right fix might be to use bintime rather than sbintime to avoid the rounding errors.  I also suspect this problem affects other emulated timers such as atpit/8254 where there is less freedom to wish away the problem with a power-of-two frequency change. Still, I committed this as-is because it was shown to fix a real-world problem and it doesn't prevent doing a better fix later. -- Ian