Re: [PATCH v2] KVM: VMX: Update instruction length on intercepted BP

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

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Gleb Natapov wrote:
> On Mon, Feb 15, 2010 at 03:53:04PM +0100, Jan Kiszka wrote:
>> We intercept #BP while in guest debugging mode. As VM exits 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.
>>
> Thinking about it some more. Why do we exit to userspace at all if we
> intercept wrong #DB? It seams to me not wise to have ability to inject
> exceptions from userspace. Exceptions generation mechanism is a part of
> CPU and we shouldn't outsource part of CPU functionality to userspace.

The guest debugging API was design to avoid maintaining a "countless"
number of breakpoints in kernel space and instead chose to loop over
user space to decide about #DB & #BP. So this part is required even if
we start thinking about an alternative interface in the future.

Jan

> 
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>> Changes in v2:
>>  - added comments
>>
>>  arch/x86/kvm/vmx.c |   13 +++++++++++++
>>  1 files changed, 13 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index f82b072..14873b9 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2775,6 +2775,12 @@ static int handle_rmode_exception(struct kvm_vcpu *vcpu,
>>  		kvm_queue_exception(vcpu, vec);
>>  		return 1;
>>  	case BP_VECTOR:
>> +		/*
>> +		 * Update instruction length as we may reinject the exception
>> +		 * from user space while in guest debugging mode.
>> +		 */
>> +		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 +2903,13 @@ static int handle_exception(struct kvm_vcpu *vcpu)
>>  		kvm_run->debug.arch.dr7 = vmcs_readl(GUEST_DR7);
>>  		/* fall through */
>>  	case BP_VECTOR:
>> +		/*
>> +		 * Update instruction length as we may reinject #BP from
>> +		 * user space while in guest debugging mode. Reading it for
>> +		 * #DB as well causes no harm, it is not used in that case.
>> +		 */
>> +		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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