Re: [PATCH 3/3] x86: KVM: VMX: preserve host's DEBUGCTLMSR_FREEZE_IN_SMM while in the guest mode

[mlevitsk@redhat.com]

On Thu, 2025-05-08 at 06:35 -0700, Sean Christopherson wrote:
> On Wed, May 07, 2025, Sean Christopherson wrote:
> > On Thu, May 01, 2025, mlevitsk@redhat.com 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...

There is one problem with this approach though: the common x86 code will still have to decide if
to set KVM_RUN_LOAD_DEBUGCTL flag.

Checking that DEBUGCTLMSR_FREEZE_IN_SMM bit of 'vcpu->arch.host_debugctl' changed is VMX specific,
because AMD doesn't have this bit, and it might even in the future have a different bit at that
position for different purpose.

I can set the KVM_RUN_LOAD_DEBUGCTL when any bit in DEBUGCTL changes instead, which should still be rare
and then SVM code can ignore the KVM_RUN_LOAD_DEBUGCTL, while VMX code will reload the VMCS field.

Is this OK?

Best regards,
	Maxim Levitsky