linux-kernel - Re: [PATCH 1/3] KVM: x86: Snapshot the host's DEBUGCTL in common x86

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Message-ID: <5380c4fd-d0bd-4a20-8fb1-1ecad76e96b2@intel.com>
Date: Wed, 26 Feb 2025 00:19:03 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 rangemachine@...il.com, whanos@...gal.fun,
 Ravi Bangoria <ravi.bangoria@....com>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/3] KVM: x86: Snapshot the host's DEBUGCTL in common x86

On 2/25/2025 2:13 AM, Sean Christopherson wrote:
> Move KVM's snapshot of DEBUGCTL to kvm_vcpu_arch and take the snapshot in
> common x86, so that SVM can also use the snapshot.
> 
> Opportunistically change the field to a u64.  While bits 63:32 are reserved
> on AMD, not mentioned at all in Intel's SDM, and managed as an "unsigned
> long" by the kernel, DEBUGCTL is an MSR and therefore a 64-bit value.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> ---
>   arch/x86/include/asm/kvm_host.h | 1 +
>   arch/x86/kvm/vmx/vmx.c          | 8 ++------
>   arch/x86/kvm/vmx/vmx.h          | 2 --
>   arch/x86/kvm/x86.c              | 1 +
>   4 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3506f497741b..02bffe6b54c8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -781,6 +781,7 @@ struct kvm_vcpu_arch {
>   	u32 pkru;
>   	u32 hflags;
>   	u64 efer;
> +	u64 host_debugctl;
>   	u64 apic_base;
>   	struct kvm_lapic *apic;    /* kernel irqchip context */
>   	bool load_eoi_exitmap_pending;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index b71392989609..729c224b72dd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1514,16 +1514,12 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
>    */
>   void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   {
> -	struct vcpu_vmx *vmx = to_vmx(vcpu);
> -
>   	if (vcpu->scheduled_out && !kvm_pause_in_guest(vcpu->kvm))
>   		shrink_ple_window(vcpu);
>   
>   	vmx_vcpu_load_vmcs(vcpu, cpu, NULL);
>   
>   	vmx_vcpu_pi_load(vcpu, cpu);
> -
> -	vmx->host_debugctlmsr = get_debugctlmsr();
>   }
>   
>   void vmx_vcpu_put(struct kvm_vcpu *vcpu)
> @@ -7458,8 +7454,8 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>   	}
>   
>   	/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
> -	if (vmx->host_debugctlmsr)
> -		update_debugctlmsr(vmx->host_debugctlmsr);
> +	if (vcpu->arch.host_debugctl)
> +		update_debugctlmsr(vcpu->arch.host_debugctl);
>   
>   #ifndef CONFIG_X86_64
>   	/*
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 8b111ce1087c..951e44dc9d0e 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -340,8 +340,6 @@ struct vcpu_vmx {
>   	/* apic deadline value in host tsc */
>   	u64 hv_deadline_tsc;
>   
> -	unsigned long host_debugctlmsr;
> -
>   	/*
>   	 * Only bits masked by msr_ia32_feature_control_valid_bits can be set in
>   	 * msr_ia32_feature_control. FEAT_CTL_LOCKED is always included
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 58b82d6fd77c..09c3d27cc01a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4991,6 +4991,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   
>   	/* Save host pkru register if supported */
>   	vcpu->arch.host_pkru = read_pkru();
> +	vcpu->arch.host_debugctl = get_debugctlmsr();
>   
>   	/* Apply any externally detected TSC adjustments (due to suspend) */
>   	if (unlikely(vcpu->arch.tsc_offset_adjustment)) {