On Fri, Aug 27, 2021, Peter Zijlstra wrote: > On Fri, Aug 27, 2021 at 02:49:50PM +0000, Sean Christopherson wrote: > > On Fri, Aug 27, 2021, Peter Zijlstra wrote: > > > On Thu, Aug 26, 2021 at 05:57:08PM -0700, Sean Christopherson wrote: > > > > Use a per-CPU pointer to track perf's guest callbacks so that KVM can set > > > > the callbacks more precisely and avoid a lurking NULL pointer dereference. > > > > > > I'm completely failing to see how per-cpu helps anything here... > > > > It doesn't help until KVM is converted to set the per-cpu pointer in flows that > > are protected against preemption, and more specifically when KVM only writes to > > the pointer from the owning CPU. > > So the 'problem' I have with this is that sane (!KVM using) people, will > still have to suffer that load, whereas with the static_call() we patch > in an 'xor %rax,%rax' and only have immediate code flow. Again, I've no objection to the static_call() approach. I didn't even see the patch until I had finished testing my series :-/ > > Ignoring static call for the moment, I don't see how the unreg side can be safe > > using a bare single global pointer. There is no way for KVM to prevent an NMI > > from running in parallel on a different CPU. If there's a more elegant solution, > > especially something that can be backported, e.g. an rcu-protected pointer, I'm > > all for it. I went down the per-cpu path because it allowed for cleanups in KVM, > > but similar cleanups can be done without per-cpu perf callbacks. > > If all the perf_guest_cbs dereferences are with preemption disabled > (IRQs disabled, IRQ context, NMI context included), then the sequence: > > WRITE_ONCE(perf_guest_cbs, NULL); > synchronize_rcu(); > > Ensures that all prior observers of perf_guest_csb will have completed > and future observes must observe the NULL value. That alone won't be sufficient, as the read side also needs to ensure it doesn't reload perf_guest_cbs between NULL checks and dereferences. But that's easy enough to solve with a READ_ONCE and maybe a helper to make it more cumbersome to use perf_guest_cbs directly. How about this for a series? 1. Use READ_ONCE/WRITE_ONCE + synchronize_rcu() to fix the underlying bug 2. Fix KVM PT interrupt handler bug 3. Kill off perf_guest_cbs usage in architectures that don't need the callbacks 4. Replace ->is_in_guest()/->is_user_mode() with ->state(), and s/get_guest_ip/get_ip 5. Implement static_call() support 6. Cleanups, if there are any 6..N KVM cleanups, e.g. to eliminate current_vcpu and share x86+arm64 callbacks
