On Fri, May 19, 2023, Chao Gao wrote: > +Pawan, could you share your thoughts on questions about FB_CLEAR? > > On Thu, May 18, 2023 at 10:33:15AM -0700, Sean Christopherson wrote: > >I do like snapshotting and then updating the value, even though there's likely no > >meaningful performance benefit, as that would provide a place to document that > >the "supported" value is dynamic. Though the fact that it's dynamic is arguably a bug > >in its own right, e.g. if userspace isn't careful, a VM can have vCPUs with different > >values for ARCH_CAPABILITIES. But fixing that is probably a fool's errand. So > > I am not sure if fixing it is fool. There would be some other problem: Heh, "fool's errand" is an idiom that means doing something has almost no chance of succeeding, not that doing something is foolish. I 100% agree that there's value in presenting a consistent model to the guest, but there are conflicting requirements in play. To present a consistent model, KVM essentially needs to disallow changing the module param after VMs/vCPUs have been created, but that would prevent userspace from toggling the param while VMs are running, e.g. in response to a new vulnerability. The only feasible idea I can think of is to disallow *disabling* the mitigation while VMs/vCPUs are active. But then that prevents turning the L1D flush mitigation back off if some other mitigation is deployed, e.g. via livepatch, policy update, etc. That's why I said trying to fix the knob is probably a fool's errand. AFAICT, there's no straightforward solution that makes everybody happy. :-/
