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Message-ID: <5e81257e-df94-271e-3aef-cab54682174e@intel.com>
Date: Wed, 27 Jan 2021 09:48:38 +0800
From: Chenyi Qiang <chenyi.qiang@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Xiaoyao Li <xiaoyao.li@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH 1/2] KVM: X86: Add support for the emulation of
DR6_BUS_LOCK bit
On 1/27/2021 12:31 AM, Paolo Bonzini wrote:
> 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
>
Will do the changes.
Thanks!
Chenyi
>> /*
>> * 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;
>>
>
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