On 18/07/2019 13:10, Jan Beulich wrote: > From: Ricardo Neri <ricardo.neri-calde...@linux.intel.com> > > Programming MTRR registers in multi-processor systems is a rather lengthy > process. Furthermore, all processors must program these registers in lock > step and with interrupts disabled; the process also involves flushing > caches and TLBs twice. As a result, the process may take a considerable > amount of time. > > On some platforms, this can lead to a large skew of the refined-jiffies > clock source. Early when booting, if no other clock is available (e.g., > booting with hpet=disabled), the refined-jiffies clock source is used to > monitor the TSC clock source. If the skew of refined-jiffies is too large, > Linux wrongly assumes that the TSC is unstable: > > clocksource: timekeeping watchdog on CPU1: Marking clocksource > 'tsc-early' as unstable because the skew is too large: > clocksource: 'refined-jiffies' wd_now: fffedc10 wd_last: > fffedb90 mask: ffffffff > clocksource: 'tsc-early' cs_now: 5eccfddebc cs_last: 5e7e3303d4 > mask: ffffffffffffffff > tsc: Marking TSC unstable due to clocksource watchdog > > As per measurements, around 98% of the time needed by the procedure to > program MTRRs in multi-processor systems is spent flushing caches with > wbinvd(). As per the Section 11.11.8 of the Intel 64 and IA 32 > Architectures Software Developer's Manual, it is not necessary to flush > caches if the CPU supports cache self-snooping. Thus, skipping the cache > flushes can reduce by several tens of milliseconds the time needed to > complete the programming of the MTRR registers: > > Platform Before After > 104-core (208 Threads) Skylake 1437ms 28ms > 2-core ( 4 Threads) Haswell 114ms 2ms > > Reported-by: Mohammad Etemadi <mohammad.etem...@intel.com> > Signed-off-by: Ricardo Neri <ricardo.neri-calde...@linux.intel.com> > [Linux commit fd329f276ecaad7a371d6f91b9bbea031d0c3440] > > Use alternatives patching instead of static_cpu_has() (which we don't > have [yet]). > Interestingly we've been lacking the 2nd wbinvd(), which I'm taking the > liberty here. > > Requested-by: Andrew Cooper <andrew.coop...@citrix.com> > Signed-off-by: Jan Beulich <jbeul...@suse.com>
Reviewed-by: Andrew Cooper <andrew.coop...@citrix.com> _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
