linux-kernel - Re: [PATCH v4 4/4] x86: KVM: VMX: preserve DEBUGCTLMSR_FREEZE_IN

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Message-ID: <5b43682cff05f0399c4d75cf6361374280f12c05.camel@redhat.com>
Date: Fri, 16 May 2025 10:49:24 -0400
From: mlevitsk@...hat.com
To: Chao Gao <chao.gao@...el.com>
Cc: kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Sean
 Christopherson <seanjc@...gle.com>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org, Ingo Molnar <mingo@...hat.com>, 
 linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>, Paolo
 Bonzini <pbonzini@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v4 4/4] x86: KVM: VMX: preserve DEBUGCTLMSR_FREEZE_IN_SMM

On Fri, 2025-05-16 at 11:39 +0800, Chao Gao wrote:
> > @@ -7368,6 +7381,9 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
> > 	if (run_flags & KVM_RUN_LOAD_GUEST_DR6)
> > 		set_debugreg(vcpu->arch.dr6, 6);
> > 
> > +	if (run_flags & KVM_RUN_LOAD_DEBUGCTL)
> > +		vmx_guest_debugctl_write(vcpu, vmx_guest_debugctl_read());
> 
> ...
> 
> > +
> > 	/*
> > 	 * Refresh vmcs.HOST_CR3 if necessary.  This must be done immediately
> > 	 * prior to VM-Enter, as the kernel may load a new ASID (PCID) any time
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index 1b80479505d3..5ddedf73392b 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -416,6 +416,8 @@ static inline void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr,
> > 
> > void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu);
> > u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated);
> > +void vmx_guest_debugctl_write(struct kvm_vcpu *vcpu, u64 val);
> > +u64 vmx_guest_debugctl_read(void);
> > 
> > /*
> >  * Note, early Intel manuals have the write-low and read-high bitmap offsets
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 684b8047e0f2..a85078dfa36d 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -10752,7 +10752,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > 		dm_request_for_irq_injection(vcpu) &&
> > 		kvm_cpu_accept_dm_intr(vcpu);
> > 	fastpath_t exit_fastpath;
> > -	u64 run_flags;
> > +	u64 run_flags, host_debug_ctl;
> > 
> > 	bool req_immediate_exit = false;
> > 
> > @@ -11024,7 +11024,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > 		set_debugreg(0, 7);
> > 	}
> > 
> > -	vcpu->arch.host_debugctl = get_debugctlmsr();
> > +	host_debug_ctl = get_debugctlmsr();
> > +	if (host_debug_ctl != vcpu->arch.host_debugctl)
> > +		run_flags |= KVM_RUN_LOAD_DEBUGCTL;
> > +	vcpu->arch.host_debugctl = host_debug_ctl;
> 
> IIUC, using run_flags here may only update the GUEST_DEBUGCTL field of a vmcs02,
> leaving vmcs01 not updated.

Hi,

Thankfully this should not be a problem because when KVM exits from vmcs02 to vmcs01 the
IA32_DEBUGCTL is reset to 0. 

If I’m not mistaken, this always happens regardless of the VM_ENTRY_LOAD_DEBUG_CONTROLS.

Now since I wrapped the write of the with a helper function (vmx_guest_debugctl_write),
it should pick up the new value.


Best regards,
	Maxim Levitsky

> 
> > 
> > 	guest_timing_enter_irqoff();
> > 
> > -- 
> > 2.46.0
> > 
> > 
>