On Fri, Feb 17, 2023, Mathias Krause wrote: > On 16.02.23 18:32, Sean Christopherson wrote: > > I'm not necessarily opposed to such aggressive optimization, but the ROI is likely > > very, very low. For optimized workloads, there simply aren't very many VM-Exits, > > e.g. the majority of exits on a modern CPU are due to timer ticks. And even those > > will hopefully be eliminiated in the not-too-distant future, e.g. by having hardware > > virtualize the TSC deadline timer, and by moving to a vCPU scheduling scheme that > > allows for a tickless host. > > Well, for guests running grsecurity kernels, there's also the CR0.WP > toggling triggering VMEXITs, which happens a lot! -- at least until > something along the lines of [1] gets merged *hint ;)* Ha! It's high on my todo list for 6.4, catching up on other stuff at the moment. That series is also _exactly_ why the ROI for aggressive cache line optimization is low. The better long term answer is almost always to avoid the VM-Exit in the first place, or failing that, to handle the exit in a fastpath. Sometimes it takes a few years, e.g. to get necessary hardware support, but x86 virtualization is fast approaching the point where anything remotely performance critical is handled entirely within the guest.
