On Fri, Apr 26, 2024, Kan Liang wrote: > > Optimization 4 > > allows the host side to immediately profiling this part instead of > > waiting for vcpu to reach to PMU context switch locations. Doing so > > will generate more accurate results. > > If so, I think the 4 is a must to have. Otherwise, it wouldn't honer the > definition of the exclude_guest. Without 4, it brings some random blind > spots, right? +1, I view it as a hard requirement. It's not an optimization, it's about accuracy and functional correctness. What _is_ an optimization is keeping guest state loaded while KVM is in its run loop, i.e. initial mediated/passthrough PMU support could land upstream with unconditional switches at entry/exit. The performance of KVM would likely be unacceptable for any production use cases, but that would give us motivation to finish the job, and it doesn't result in random, hard to diagnose issues for userspace. > > Do we want to preempt that? I think it depends. For regular cloud > > usage, we don't. But for any other usages where we want to prioritize > > KVM/VMM profiling over guest vPMU, it is useful. > > > > My current opinion is that optimization 4 is something nice to have. > > But we should allow people to turn it off just like we could choose to > > disable preempt kernel. > > The exclude_guest means everything but the guest. I don't see a reason > why people want to turn it off and get some random blind spots.
