GOn Wed, Jan 18, 2017 at 12:37:25PM -0200, Marcelo Tosatti wrote: > On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote: > > > > > > On 18/01/2017 13:24, Marcelo Tosatti wrote: > > > 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: > > >>>>> 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. > > >> > > >> 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. > > > > It's just that it's not obvious why you get better results with biased > > host timestamps. What makes the biased host timestamp more precise? > > > > I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony > > does not support it---but I would still prefer you to support > > PTP_SYS_OFFSET_PRECISE as well. > > A single TSC read could be used to implement the PRECISE ioctl, but if > a timer interrupt takes place on either the host or the guest, and that > timer interrupt "adds" the TSC delta to xtime.nsec/xtime.sec, then that > single TSC read cannot be used. > > So you would have to stop timer interrupts (in guest and host) for the duration of the > PRECISE ioctl in the guest to avoid that situation, which seems a bit > overkill to me. > > Any other ideas? Could have a hypercall that disables host timer interrupts for a specified amount of time... But that does not scale with multiple VMs. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
