On Wed, Jan 18, 2017 at 04:17:18PM +0000, Punit Agrawal wrote: > Mark Rutland <mark.rutland@xxxxxxx> writes: > > > On Wed, Jan 18, 2017 at 02:51:31PM +0000, Punit Agrawal wrote: > >> I should've clarified in my reply that I wasn't looking to support the > >> third instance from Mark's examples above - "monitor all vCPUs on a > >> pCPU". I think it'll be quite expensive to figure out which threads from > >> a given pool are vCPUs. > > > > I'm not sure I follow why you would need to do that? > > > > In that case, we'd open a CPU-bound perf event for the pCPU, which would > > get installed in the CPU context immediately. It would be present for > > all tasks. > > > > Given it's present for all tasks, we don't need to figure out which > > happen to have vCPUs. The !vCPU tasks simply shouldn't trigger events. > > > > Am I missing something? > > When enabling a CPU-bound event for pCPU, we'd have to enable trapping > of TLB operations for the vCPUs running on pCPU. Have a look at Patch > 7/9. > > Also, we'd have to enable/disable trapping when tasks are migrated > between pCPUs. Ah, so we can't configure the trap and leave it active, since it'll affect the host. We could have a per-cpu flag, and a hook into vcpu_run, but that's also gnarly. I'll have a think. > So far I've assumed that a VM pid is immutable. If that doesn't hold > then we need to think of another mechanism to refer to a VM from > userspace. Even if we can't migrate the VM between processes (i.e. it's immutable), it's still not unique within a process, so I'm fairly sure we need another mechanism (even if we get away with the common case today). Thanks, Mark. -- 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
