Re: [patch 3/3] PTP: add kvm PTP driver

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On Wed, Jan 18, 2017 at 10:17:38AM -0200, Marcelo Tosatti wrote:
> On Tue, Jan 17, 2017 at 04:36:21PM +0100, Radim Krcmar wrote:
> > 2017-01-17 09:30-0200, Marcelo Tosatti:
> > > On Tue, Jan 17, 2017 at 09:03:27AM +0100, Miroslav Lichvar wrote:
> > >> On Mon, Jan 16, 2017 at 06:01:14PM -0200, Marcelo Tosatti wrote:
> > >> > On Mon, Jan 16, 2017 at 05:47:15PM -0200, Marcelo Tosatti wrote:
> > >> > > On Mon, Jan 16, 2017 at 05:36:55PM -0200, Marcelo Tosatti wrote:
> > >> > > > Sorry, unless i am misunderstanding how this works, it'll get the guest clock
> > >> > > > 2us behind, which is something not wanted.
> > >> > > > 
> > >> > > > Miroslav, if ->gettime64 returns the host realtime at 2us in the past, 
> > >> > > > this means Chrony will sync the guest clock to
> > >> > > > 
> > >> > > > host realtime - 2us
> > >> > > > 
> > >> > > > Is that correct?
> > >> 
> > >> Probably. It depends on the error of both host and guest timestamps.
> > >> If the error is the same on both sides, it will cancel out. An
> > >> occasional spike in the delay shouldn't be a problem as the reading
> > >> will be filtered out, but for best accuracy it's necessary that the
> > >> host's timestamp is taken in the middle between the guest's
> > >> timestamps.
> > > 
> > > The problem is that spikes can be far from occasional: it depends on activity of
> > > the host CPU and interrupts. Whose delay can be "intermittent": as long
> > > as interrupts are being sent to the host CPU, for example, the delay
> > > will be high (which can last minutes).
> > > 
> > > The TSC reading in the guest KVM PTP driver corrects for that delay.
> > > 
> > >> Users of the PTP_SYS_OFFSET ioctl assume that (ts[0]+ts[2])/2
> > >> corresponds to ts[1], (ts[2]+ts[4])/2 corresponds to ts[3], and so on.
> > >> 
> > >>                     ts[1]     ts[3]
> > >> Host time    ---------+---------+........
> > >>                       |         |
> > >>                       |         |
> > >> Guest time   ----+---------+---------+......
> > >>                 ts[0]    ts[2]     ts[4]
> > 
> > KVM PTP delay moves host ts[i] to be close to guest ts[i+1] and makes
> > the offset very consistent, so the graph would look like:
> > 
> >                         ts[1]     ts[3]
> > Host time    -------------+---------+........
> >                           |         |
> >                           |         |
> > Guest time   ----+---------+---------+......
> >                 ts[0]    ts[2]     ts[4]
> > 
> > which doesn't sound good if users assume that the host reading is in the
> > middle -- the guest time would be ahead of the host time.
> > 
> > I'm wondering why is the PTP precision around 10ns, when the hypercall
> > takes around 2-3k cycles.  Have you measured the guest<->host offset by
> > getting the output of the hypercall, i.e.
> >   {host_sec @ tsc, host_nsec @ tsc, tsc}
> > and comparing it with guest time computed from the same tsc, i.e.
> >   {guest_sec @ tsc, guest_nsec @ tsc}
> > ?
> > 
> > Thanks.
> 
> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating
> intense interrupts). Follows results:
> 
> Without TSC delta calculation:
> =============================
> 
> #* PHC0                          0   3   377     2    -99ns[ +206ns] +/-  116ns
> #* PHC0                          0   3   377     8   +202ns[ +249ns] +/-  111ns
> #* PHC0                          0   3   377     8   -213ns[ +683ns] +/-   88ns
> #* PHC0                          0   3   377     6    +77ns[ +319ns] +/-   56ns
> #* PHC0                          0   3   377     4   -771ns[-1029ns] +/-   93ns
> #* PHC0                          0   3   377    10    -49ns[  -58ns] +/-  121ns
> #* PHC0                          0   3   377     9   +562ns[ +703ns] +/-  107ns
> #* PHC0                          0   3   377     6     -2ns[   -3ns] +/-   94ns
> #* PHC0                          0   3   377     4   +451ns[ +494ns] +/-  138ns
> #* PHC0                          0   3   377    11    -67ns[  -74ns] +/-  113ns
> #* PHC0                          0   3   377     8   +244ns[ +264ns] +/-  119ns
> #* PHC0                          0   3   377     7   -696ns[ -890ns] +/-   89ns
> #* PHC0                          0   3   377     4   +468ns[ +560ns] +/-  110ns
> #* PHC0                          0   3   377    11   -310ns[ -430ns] +/-   72ns
> #* PHC0                          0   3   377     9   +189ns[ +298ns] +/-   54ns
> #* PHC0                          0   3   377     7   +594ns[ +473ns] +/-   96ns
> #* PHC0                          0   3   377     5   +151ns[ +280ns] +/-   71ns
> #* PHC0                          0   3   377    10   -590ns[ -696ns] +/-   94ns
> #* PHC0                          0   3   377     8   +415ns[ +526ns] +/-   74ns
> #* PHC0                          0   3   377     6  +1381ns[+1469ns] +/-  101ns
> #* PHC0                          0   3   377     4   +571ns[+1304ns] +/-   54ns
> #* PHC0                          0   3   377     8     -5ns[  +71ns] +/-  139ns
> #* PHC0                          0   3   377     7   -247ns[ -502ns] +/-   69ns
> #* PHC0                          0   3   377     5   -283ns[ +879ns] +/-   73ns
> #* PHC0                          0   3   377     3   +148ns[ -109ns] +/-   61ns
> 
> With TSC delta calculation:
> ============================
> 
> #* PHC0                          0   3   377     7   +379ns[ +432ns] +/-   53ns
> #* PHC0                          0   3   377     9   +106ns[ +420ns] +/-   42ns
> #* PHC0                          0   3   377     7    -58ns[ -136ns] +/-   62ns
> #* PHC0                          0   3   377    12    +93ns[  -38ns] +/-   64ns
> #* PHC0                          0   3   377     8    +84ns[ +107ns] +/-   69ns
> #* PHC0                          0   3   377     3    -76ns[ -103ns] +/-   52ns
> #* PHC0                          0   3   377     7    +52ns[  +63ns] +/-   50ns
> #* PHC0                          0   3   377    11    +29ns[  +31ns] +/-   70ns
> #* PHC0                          0   3   377     7    -47ns[  -56ns] +/-   42ns
> #* PHC0                          0   3   377    10    -35ns[  -42ns] +/-   33ns
> #* PHC0                          0   3   377     7    -32ns[  -34ns] +/-   42ns
> #* PHC0                          0   3   377    11   -172ns[ -173ns] +/-  118ns
> #* PHC0                          0   3   377     6    +65ns[  +76ns] +/-   23ns
> #* PHC0                          0   3   377     9    +18ns[  +23ns] +/-   37ns
> #* PHC0                          0   3   377     6    +41ns[  -60ns] +/-   30ns
> #* PHC0                          0   3   377    10    +39ns[ +183ns] +/-   42ns
> #* PHC0                          0   3   377     6    +50ns[ +102ns] +/-   86ns
> #* PHC0                          0   3   377    11    +50ns[  +75ns] +/-   52ns
> #* PHC0                          0   3   377     6    +50ns[ +116ns] +/-  100ns
> #* PHC0                          0   3   377    10    +46ns[  +65ns] +/-   79ns
> #* PHC0                          0   3   377     7    -38ns[  -51ns] +/-   29ns
> #* PHC0                          0   3   377    10    -11ns[  -12ns] +/-   32ns
> #* PHC0                          0   3   377     7    -31ns[  -32ns] +/-   99ns
> #* PHC0                          0   3   377    10   +222ns[ +238ns] +/-   58ns
> #* PHC0                          0   3   377     6   +185ns[ +207ns] +/-   39ns
> #* PHC0                          0   3   377    10   -392ns[ -394ns] +/-  118ns
> #* PHC0                          0   3   377     6     -9ns[  -50ns] +/-   35ns
> #* PHC0                          0   3   377    10   -346ns[ -355ns] +/-  111ns
> 
> 
> Do you still want to drop it in favour of simplicity?
> 

This is the output of "chronyc sources". See section "Time sources"
of https://chrony.tuxfamily.org/doc/2.4/chronyc.html.


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