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

Paolo Bonzini
2021-01-26T18:08:47Z

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;
>