Re: [RESEND PATCH 1/2] KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/01/21 07:49, Chenyi Qiang wrote:
> To avoid breaking the CPUs without bus lock detection, activate the
> DR6_BUS_LOCK bit (bit 11) conditionally in DR6_FIXED_1 bits.
> 
> The set/clear of DR6_BUS_LOCK is similar to the DR6_RTM in DR6
> register. The processor clears DR6_BUS_LOCK when bus lock debug
> exception is generated. (For all other #DB the processor sets this bit
> to 1.) Software #DB handler should set this bit before returning to the
> interrupted task.
> 
> For VM exit caused by debug exception, bit 11 of the exit qualification
> is set to indicate that a bus lock debug exception condition was
> detected. The VMM should emulate the exception by clearing bit 11 of the
> guest DR6.

Please rename DR6_INIT to DR6_ACTIVE_LOW, and then a lot of changes 
become simpler:

> -		dr6 |= DR6_BD | DR6_RTM;
> +		dr6 |= DR6_BD | DR6_RTM | DR6_BUS_LOCK;

dr6 |= DR6_BD | DR6_ACTIVE_LOW;

>   		ctxt->ops->set_dr(ctxt, 6, dr6);
>   		return emulate_db(ctxt);
>   	}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cce0143a6f80..3d8a0e30314f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1860,7 +1860,7 @@ static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
>   	get_debugreg(vcpu->arch.db[2], 2);
>   	get_debugreg(vcpu->arch.db[3], 3);
>   	/*
> -	 * We cannot reset svm->vmcb->save.dr6 to DR6_FIXED_1|DR6_RTM here,
> +	 * We cannot reset svm->vmcb->save.dr6 to DR6_FIXED_1|DR6_RTM|DR6_BUS_LOCK here,

We cannot reset svm->vmcb->save.dr6 to DR6_ACTIVE_LOW

>   	 * because db_interception might need it.  We can do it before vmentry.
>   	 */
>   	vcpu->arch.dr6 = svm->vmcb->save.dr6;
> @@ -1911,7 +1911,7 @@ static int db_interception(struct vcpu_svm *svm)
>   	if (!(svm->vcpu.guest_debug &
>   	      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
>   		!svm->nmi_singlestep) {
> -		u32 payload = (svm->vmcb->save.dr6 ^ DR6_RTM) & ~DR6_FIXED_1;
> +		u32 payload = (svm->vmcb->save.dr6 ^ (DR6_RTM|DR6_BUS_LOCK)) & ~DR6_FIXED_1;

u32 payload = svm->vmcb->save.dr6 ^ DR6_ACTIVE_LOW;

>   		kvm_queue_exception_p(&svm->vcpu, DB_VECTOR, payload);
>   		return 1;
>   	}
> @@ -3778,7 +3778,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>   	if (unlikely(svm->vcpu.arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
>   		svm_set_dr6(svm, vcpu->arch.dr6);
>   	else
> -		svm_set_dr6(svm, DR6_FIXED_1 | DR6_RTM);
> +		svm_set_dr6(svm, DR6_FIXED_1 | DR6_RTM | DR6_BUS_LOCK);

svm_set_dr6(svm, DR6_ACTIVE_LOW);

>   
>   	clgi();
>   	kvm_load_guest_xsave_state(vcpu);
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index e2f26564a12d..c5d71a9b3729 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -412,7 +412,7 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned long *exit
>   			if (!has_payload) {
>   				payload = vcpu->arch.dr6;
>   				payload &= ~(DR6_FIXED_1 | DR6_BT);
> -				payload ^= DR6_RTM;
> +				payload ^= DR6_RTM | DR6_BUS_LOCK;

payload &= ~DR6_BT;
payload ^= DR6_ACTIVE_LOW;

>   			}
>   			*exit_qual = payload;
>   		} else
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3f7c1fc7a3ce..06de2b9e57f3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -483,19 +483,20 @@ void kvm_deliver_exception_payload(struct kvm_vcpu *vcpu)
>   		 */
>   		vcpu->arch.dr6 &= ~DR_TRAP_BITS;
>   		/*
> -		 * DR6.RTM is set by all #DB exceptions that don't clear it.
> +		 * DR6.RTM and DR6.BUS_LOCK are set by all #DB exceptions
> +		 * that don't clear it.
>   		 */
> -		vcpu->arch.dr6 |= DR6_RTM;
> +		vcpu->arch.dr6 |= DR6_RTM | DR6_BUS_LOCK;
>   		vcpu->arch.dr6 |= payload;
>   		/*
> -		 * Bit 16 should be set in the payload whenever the #DB
> -		 * exception should clear DR6.RTM. This makes the payload
> -		 * compatible with the pending debug exceptions under VMX.
> -		 * Though not currently documented in the SDM, this also
> -		 * makes the payload compatible with the exit qualification
> -		 * for #DB exceptions under VMX.
> +		 * Bit 16/Bit 11 should be set in the payload whenever
> +		 * the #DB exception should clear DR6.RTM/DR6.BUS_LOCK.
> +		 * This makes the payload compatible with the pending debug
> +		 * exceptions under VMX. Though not currently documented in
> +		 * the SDM, this also makes the payload compatible with the
> +		 * exit qualification for #DB exceptions under VMX.
>   		 */
> -		vcpu->arch.dr6 ^= payload & DR6_RTM;
> +		vcpu->arch.dr6 ^= payload & (DR6_RTM | DR6_BUS_LOCK);

vcpu->arch.dr6 &= ~DR_TRAP_BITS;
vcpu->arch.dr6 |= DR6_ACTIVE_LOW;
vcpu->arch.dr6 |= payload;
vcpu->arch.dr6 ^= payload & DR6_ACTIVE_LOW;

(with comments :))

and so on.

Thanks!

Paolo

>   
>   		/*
>   		 * The #DB payload is defined as compatible with the 'pending
> @@ -1126,6 +1127,9 @@ static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
>   
>   	if (!guest_cpuid_has(vcpu, X86_FEATURE_RTM))
>   		fixed |= DR6_RTM;
> +
> +	if (!guest_cpuid_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
> +		fixed |= DR6_BUS_LOCK;
>   	return fixed;
>   }
>   
> @@ -7197,7 +7201,8 @@ static int kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu)
>   	struct kvm_run *kvm_run = vcpu->run;
>   
>   	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> -		kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
> +		kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM |
> +					  DR6_BUS_LOCK;
>   		kvm_run->debug.arch.pc = kvm_get_linear_rip(vcpu);
>   		kvm_run->debug.arch.exception = DB_VECTOR;
>   		kvm_run->exit_reason = KVM_EXIT_DEBUG;
> @@ -7241,7 +7246,8 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
>   					   vcpu->arch.eff_db);
>   
>   		if (dr6 != 0) {
> -			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM;
> +			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM |
> +						  DR6_BUS_LOCK;
>   			kvm_run->debug.arch.pc = eip;
>   			kvm_run->debug.arch.exception = DB_VECTOR;
>   			kvm_run->exit_reason = KVM_EXIT_DEBUG;
>