linux-kernel - Re: [PATCH 3/3] x86: KVM: VMX: preserve host's DEBUGCTLMSR_FREEZE_IN

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Message-ID: <aByzGilzBiTa-43C@google.com>
Date: Thu, 8 May 2025 06:35:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: mlevitsk@...hat.com
Cc: kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/3] x86: KVM: VMX: preserve host's DEBUGCTLMSR_FREEZE_IN_SMM
 while in the guest mode

On Wed, May 07, 2025, Sean Christopherson wrote:
> On Thu, May 01, 2025, mlevitsk@...hat.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...

View attachment "0001-KVM-x86-Convert-vcpu_run-s-immediate-exit-param-into.patch" of type "text/x-diff" (6712 bytes)

View attachment "0002-KVM-x86-Drop-kvm_x86_ops.set_dr6-in-favor-of-a-new-K.patch" of type "text/x-diff" (5678 bytes)