Re: [PATCH v2] KVM: SVM: Add Bus Lock Detect support

[Shivansh Dhiman <shivansh.dhiman@amd.com>]

Hi,

Gentle ping for reviewing this patch for Bus Lock Detect. Would appreciate
any feedback or guidance on next steps.

Thanks,
Shivansh


On 21-11-25 13:42, Shivansh Dhiman wrote:
> From: Ravi Bangoria <ravi.bangoria@amd.com>
> 
> Add Bus Lock Detect support in AMD SVM. Bus Lock Detect is enabled through
> MSR_IA32_DEBUGCTLMSR and MSR_IA32_DEBUGCTLMSR is virtualized only if LBR
> Virtualization is enabled. Add this dependency in the SVM.
> 
> While adding Bus Lock Detect support, also fix DR6 handling in nested
> virtualization. Using DR6_FIXED_1 to prevent reset of BLD bit (bit 11)
> between VMRUNs. However, it preserves DR6_RTM, which is a reserved bit
> on AMD processors. So, DR6_RTM bit must always be set to 1.
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> Co-developed-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
> ---
> Changelog:
> v1 --> v2
>  * Rebased and used guest_cpu_cap_has() instead of guest_cpuid_has().
> 
>  v1: https://lore.kernel.org/all/20240808062937.1149-5-ravi.bangoria@amd.com
> ---
>  arch/x86/kvm/svm/nested.c |  3 ++-
>  arch/x86/kvm/svm/svm.c    | 17 ++++++++++++++++-
>  arch/x86/kvm/svm/svm.h    |  2 +-
>  3 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index da6e80b3ac35..5af7fbf5c536 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -665,7 +665,8 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
>  	/* These bits will be set properly on the first execution when new_vmc12 is true */
>  	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_DR))) {
>  		vmcb02->save.dr7 = svm->nested.save.dr7 | DR7_FIXED_1;
> -		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_ACTIVE_LOW;
> +		/* DR6_RTM is a reserved bit on AMD and as such must be set to 1 */
> +		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_FIXED_1 | DR6_RTM;
>  		vmcb_mark_dirty(vmcb02, VMCB_DR);
>  	}
>  
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 10c21e4c5406..ceef5b697e11 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -831,6 +831,9 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
>  			    (is_guest_mode(vcpu) && guest_cpu_cap_has(vcpu, X86_FEATURE_LBRV) &&
>  			    (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK));
>  
> +	/* Bus Lock Detect depends on LBR Virtualization */
> +	enable_lbrv |= (svm->vmcb->save.dbgctl & DEBUGCTLMSR_BUS_LOCK_DETECT);
> +
>  	if (enable_lbrv && !current_enable_lbrv)
>  		__svm_enable_lbrv(vcpu);
>  	else if (!enable_lbrv && current_enable_lbrv)
> @@ -2984,6 +2987,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  			data &= ~DEBUGCTLMSR_BTF;
>  		}
>  
> +		if ((data & DEBUGCTLMSR_BUS_LOCK_DETECT) &&
> +		    !guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
> +			return 1;
> +
>  		if (data & DEBUGCTL_RESERVED_BITS)
>  			return 1;
>  
> @@ -5258,8 +5265,16 @@ static __init void svm_set_cpu_caps(void)
>  	 * Clear capabilities that are automatically configured by common code,
>  	 * but that require explicit SVM support (that isn't yet implemented).
>  	 */
> -	kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
>  	kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
> +
> +	/*
> +	 * LBR Virtualization must be enabled to support BusLockTrap inside the
> +	 * guest, since BusLockTrap is enabled through MSR_IA32_DEBUGCTLMSR and
> +	 * MSR_IA32_DEBUGCTLMSR is virtualized only if LBR Virtualization is
> +	 * enabled.
> +	 */
> +	if (!lbrv)
> +		kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
>  }
>  
>  static __init int svm_hardware_setup(void)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index c856d8e0f95e..8e8d31788c9c 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -675,7 +675,7 @@ BUILD_SVM_MSR_BITMAP_HELPERS(bool, test, test)
>  BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear)
>  BUILD_SVM_MSR_BITMAP_HELPERS(void, set, __set)
>  
> -#define DEBUGCTL_RESERVED_BITS (~DEBUGCTLMSR_LBR)
> +#define DEBUGCTL_RESERVED_BITS (~(DEBUGCTLMSR_LBR | DEBUGCTLMSR_BUS_LOCK_DETECT))
>  
>  /* svm.c */
>  extern bool dump_invalid_vmcb;