On Fri, Dec 27, 2019 at 03:03:57PM +0000, Andrew Cooper wrote: > On 24/12/2019 12:44, Roger Pau Monne wrote: > > If the flush mask matches the mask of online CPUs use the APIC ALLBUT > > destination shorthand in order to send an IPI to all CPUs on the > > system except the current one. This can only be safely used when no > > CPU hotplug or unplug operations are taking place, no offline CPUs or > > those have been onlined and parked and finally when all CPUs in the > > system have been accounted for (ie: the number of CPUs doesn't exceed > > NR_CPUS and APIC IDs are below MAX_APICS). > > > > This is specially beneficial when using the PV shim, since using the > > shorthand avoids performing an APIC register write (or multiple ones > > if using xAPIC mode) for each destination in the flush mask. > > > > The lock time on a 32 vCPU guest using the shim without the shorthand > > is: > > > > Global lock flush_lock: addr=ffff82d0804b21c0, lockval=f602f602, not locked > > lock:228455938(79406065573135), block:205908580(556416605761539) > > > > Average lock time: 347577ns > > > > While the same guest using the shorthand: > > > > Global lock flush_lock: addr=ffff82d0804b41c0, lockval=d9c4d9bc, cpu=12 > > lock:1890775(416719148054), block:1663958(2500161282949) > > > > Average lock time: 220395ns > > > > Approximately a 1/3 improvement in the lock time. > > > > Signed-off-by: Roger Pau Monné <roger....@citrix.com> > > While these are good stats, I'm somewhat hesitant about hacking this in > like this. For one, it is a substantial amount of ad-hoc logic in > flush_area_mask() > > Shorthand safety really should be part of the apic driver, not part of > the TLB logic.
Yes, I've coded it this way because there are already similar APIC hooks (ie: send_IPI_self). I can merge the shorthand functionality with send_IPI_mask if that's preferred. Thanks, Roger. _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
