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

[Gleb Natapov <gleb@redhat.com>]

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.

> 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.