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

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 15, 2025, Maxim Levitsky wrote:
> Pass through the host's DEBUGCTL.DEBUGCTLMSR_FREEZE_IN_SMM to the guest
> GUEST_IA32_DEBUGCTL without the guest seeing this value.
> 
> Note that in the future we might allow the guest to set this bit as well,
> when we implement PMU freezing on VM own, virtual SMM entry.
> 
> Since the value of the host DEBUGCTL can in theory change between VM runs,
> check if has changed, and if yes, then reload the GUEST_IA32_DEBUGCTL with
> the new value of the host portion of it (currently only the
> DEBUGCTLMSR_FREEZE_IN_SMM bit)

No, it can't.  DEBUGCTLMSR_FREEZE_IN_SMM can be toggled via IPI callback, but
IRQs are disabled for the entirety of the inner run loop.  And if I'm somehow
wrong, this change movement absolutely belongs in a separate patch.

> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/svm/svm.c |  2 ++
>  arch/x86/kvm/vmx/vmx.c | 28 +++++++++++++++++++++++++++-
>  arch/x86/kvm/x86.c     |  2 --
>  3 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cc1c721ba067..fda0660236d8 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4271,6 +4271,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
>  	svm->vmcb->save.rsp = vcpu->arch.regs[VCPU_REGS_RSP];
>  	svm->vmcb->save.rip = vcpu->arch.regs[VCPU_REGS_RIP];
>  
> +	vcpu->arch.host_debugctl = get_debugctlmsr();
> +
>  	/*
>  	 * Disable singlestep if we're injecting an interrupt/exception.
>  	 * We don't want our modified rflags to be pushed on the stack where
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c9208a4acda4..e0bc31598d60 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2194,6 +2194,17 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
>  	return debugctl;
>  }
>  
> +static u64 vmx_get_host_preserved_debugctl(struct kvm_vcpu *vcpu)

No, just open code handling DEBUGCTLMSR_FREEZE_IN_SMM, or make it a #define.
I'm not remotely convinced that we'll ever want to emulate DEBUGCTLMSR_FREEZE_IN_SMM,
and trying to plan for that possibility and adds complexity for no immediate value.

> +{
> +	/*
> +	 * Bits of host's DEBUGCTL that we should preserve while the guest is
> +	 * running.
> +	 *
> +	 * Some of those bits might still be emulated for the guest own use.
> +	 */
> +	return DEBUGCTLMSR_FREEZE_IN_SMM;
>
>  u64 vmx_get_guest_debugctl(struct kvm_vcpu *vcpu)
>  {
>  	return to_vmx(vcpu)->msr_ia32_debugctl;
> @@ -2202,9 +2213,11 @@ u64 vmx_get_guest_debugctl(struct kvm_vcpu *vcpu)
>  static void __vmx_set_guest_debugctl(struct kvm_vcpu *vcpu, u64 data)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	u64 host_mask = vmx_get_host_preserved_debugctl(vcpu);
>  
>  	vmx->msr_ia32_debugctl = data;
> -	vmcs_write64(GUEST_IA32_DEBUGCTL, data);
> +	vmcs_write64(GUEST_IA32_DEBUGCTL,
> +		     (vcpu->arch.host_debugctl & host_mask) | (data & ~host_mask));
>  }
>  
>  bool vmx_set_guest_debugctl(struct kvm_vcpu *vcpu, u64 data, bool host_initiated)
> @@ -2232,6 +2245,7 @@ bool vmx_set_guest_debugctl(struct kvm_vcpu *vcpu, u64 data, bool host_initiated
>  	return true;
>  }
>  
> +

Spurious newline.

>  /*
>   * Writes msr value into the appropriate "register".
>   * Returns 0 on success, non-0 otherwise.
> @@ -7349,6 +7363,7 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	unsigned long cr3, cr4;
> +	u64 old_debugctl;
>  
>  	/* Record the guest's net vcpu time for enforced NMI injections. */
>  	if (unlikely(!enable_vnmi &&
> @@ -7379,6 +7394,17 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>  		vmcs_write32(PLE_WINDOW, vmx->ple_window);
>  	}
>  
> +	old_debugctl = vcpu->arch.host_debugctl;
> +	vcpu->arch.host_debugctl = get_debugctlmsr();
> +
> +	/*
> +	 * In case the host DEBUGCTL had changed since the last time we
> +	 * read it, update the guest's GUEST_IA32_DEBUGCTL with
> +	 * the host's bits.
> +	 */
> +	if (old_debugctl != vcpu->arch.host_debugctl)

This can and should be optimized to only do an update if a host-preserved bit
is toggled.

> +		__vmx_set_guest_debugctl(vcpu, vmx->msr_ia32_debugctl);

I would rather have a helper that explicitly writes the VMCS field, not one that
sets the guest value *and* writes the VMCS field.

The usage in init_vmcs() doesn't need to write vmx->msr_ia32_debugctl because the
vCPU is zero allocated, and this usage doesn't change vmx->msr_ia32_debugctl.
So the only path that actually needs to modify vmx->msr_ia32_debugctl is
vmx_set_guest_debugctl().

> +
>  	/*
>  	 * We did this in prepare_switch_to_guest, because it needs to
>  	 * be within srcu_read_lock.
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 844e81ee1d96..05e866ed345d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11020,8 +11020,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		set_debugreg(0, 7);
>  	}
>  
> -	vcpu->arch.host_debugctl = get_debugctlmsr();
> -
>  	guest_timing_enter_irqoff();
>  
>  	for (;;) {
> -- 
> 2.26.3
>