Date: Wed, 4 Mar 2015 21:24:22 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Ryan Stone <rysto32@gmail.com> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, Bryan Venteicher <bryanv@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org> Subject: Re: svn commit: r278183 - in head/sys: amd64/include conf dev/xen/timer i386/include i386/xen x86/include x86/x86 Message-ID: <20150304192421.GL2379@kib.kiev.ua> In-Reply-To: <CAFMmRNxSJ554uyjzc5-NW%2Bjw_MpH_w_Rvt7x9i6zkFLBxenevw@mail.gmail.com> References: <201502040826.t148Qi8W096721@svn.freebsd.org> <CAFMmRNxSJ554uyjzc5-NW%2Bjw_MpH_w_Rvt7x9i6zkFLBxenevw@mail.gmail.com>
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On Wed, Mar 04, 2015 at 01:24:52PM -0500, Ryan Stone wrote: > On Wed, Feb 4, 2015 at 3:26 AM, Bryan Venteicher <bryanv@freebsd.org> wrote: > > +static void > > +pvclock_read_time_info(struct pvclock_vcpu_time_info *ti, > > + uint64_t *cycles, uint8_t *flags) > > +{ > > + uint32_t version; > > + > > + do { > > + version = ti->version; > > + rmb(); > > + *cycles = ti->system_time + pvclock_get_nsec_offset(ti); > > + *flags = ti->flags; > > + rmb(); > > + } while ((ti->version & 1) != 0 || ti->version != version); > > +} > > The Linux implementation of this does a "rdtsc_barrier();" in place of > the rmb(), and in some situations that appears to result in a full > mfence, not just an lfence (the code is macro soup so it's difficult > to understand what's happening). Are you sure that the rmb() is > enough? I believe that the concern is whether the rdtsc() itself is > allowed to be re-ordered around an lfence. > > This stack overflow answer seems to indicate the a full mfence is > necessary on AMD processors: > > http://stackoverflow.com/questions/12631856/difference-between-rdtscp-rdtsc-memory-and-cpuid-rdtsc Yes, Intel declares that lfence is enough, but for AMD the mfence should be used. See r238973/r238975 and sys/x86/x86/tsc.c.
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