On Wed, May 07, 2025, Sean Christopherson wrote: > On Thu, May 01, 2025, mlevitsk@xxxxxxxxxx wrote: > > Any ideas on how to solve this then? Since currently its the common code that > > reads the current value of the MSR_IA32_DEBUGCTLMSR and it doesn't leave any > > indication about if it changed I can do either > > > > 1. store old value as well, something like 'vcpu->arch.host_debugctl_old' Ugly IMHO. > > > > 2. add DEBUG_CTL to the set of the 'dirty' registers, e.g add new bit for kvm_register_mark_dirty > > It looks a bit overkill to me > > > > 3. Add new x86 callback for something like .sync_debugctl(). I vote for this option. > > > > What do you think/prefer? > > I was going to say #3 as well, but I think I have a better idea. > > DR6 has a similar problem; the guest's value needs to be loaded into hardware, > but only somewhat rarely, and more importantly, never on a fastpath reentry. > > Forced immediate exits also have a similar need: some control logic in common x86 > needs instruct kvm_x86_ops.vcpu_run() to do something. > > Unless I've misread the DEBUGCTLMSR situation, in all cases, common x86 only needs > to a single flag to tell vendor code to do something. The payload for that action > is already available. > > So rather than add a bunch of kvm_x86_ops hooks that are only called immediately > before kvm_x86_ops.vcpu_run(), expand @req_immediate_exit into a bitmap of flags > to communicate what works needs to be done, without having to resort to a field > in kvm_vcpu_arch that isn't actually persistent. > > The attached patches are relatively lightly tested, but the DR6 tests from the > recent bug[*] pass, so hopefully they're correct? > > The downside with this approach is that it would be difficult to backport to LTS > kernels, but given how long this has been a problem, I'm not super concerned about > optimizing for backports. > > If they look ok, feel free to include them in the next version. Or I can post > them separately if you want. And of course I forgot to attach the patches...
