Re: [PATCH] KVM: VMX: Update instruction length on intercepted BP

[Jan Kiszka <jan.kiszka@web.de>]

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Gleb Natapov wrote:
> On Sat, Feb 13, 2010 at 10:31:12AM +0100, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> We intercept #BP while in guest debugging mode. As VM exists due to
>> intercepted exceptions do not necessarily come with valid
>> idt_vectoring, we have to update event_exit_inst_len explicitly in such
>> cases. At least in the absence of migration, this ensures that
>> re-injections of #BP will find and use the correct instruction length.
>>
> event_exit_inst_len is only used for event reinjection. Since event
> intercepted here will not be reinjected why updating event_exit_inst_len
> is needed here?

In guest debugging mode a #BP exception is always reported to user space
to find out what caused it. If it was the guest itself, the exception is
reinjected, on older kernels via KVM_SET_GUEST_DEBUG and since 2.6.33
via KVM_SET_VCPU_EVENTS (the latter requires some qemu patch that I will
post later).

As we currently do not update event_exit_inst_len on #BP exits,
reinjecting fails unless event_exit_inst_len happens to be 1 from some
other exit.

Jan

> 
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>> Please consider for stable, this fixes guest debugging scenarios.
>>
>>  arch/x86/kvm/vmx.c |    4 ++++
>>  1 files changed, 4 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index f82b072..e9f64e8 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2775,6 +2775,8 @@ static int handle_rmode_exception(struct kvm_vcpu *vcpu,
>>  		kvm_queue_exception(vcpu, vec);
>>  		return 1;
>>  	case BP_VECTOR:
>> +		to_vmx(vcpu)->vcpu.arch.event_exit_inst_len =
>> +			vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
>>  		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
>>  			return 0;
>>  		/* fall through */
>> @@ -2897,6 +2899,8 @@ static int handle_exception(struct kvm_vcpu *vcpu)
>>  		kvm_run->debug.arch.dr7 = vmcs_readl(GUEST_DR7);
>>  		/* fall through */
>>  	case BP_VECTOR:
>> +		vmx->vcpu.arch.event_exit_inst_len =
>> +			vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
>>  		kvm_run->exit_reason = KVM_EXIT_DEBUG;
>>  		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>>  		kvm_run->debug.arch.exception = ex_no;
>>
> 
> 
> 
> --
> 			Gleb.



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