Re: [Regression] kvm-userspace: VM freezes after booting FreeDOS

[Avi Kivity <avi@qumranet.com>]

Jan Kiszka wrote:
> It looks like that we should forward all #DB exceptions to the guest in
> real mode unless we are sure they were caused by a host-injection. Here
> is more or less a hack to achieve this (breaking guest debugging for
> now):
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b60fcec..a6f9c9b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2244,6 +2244,15 @@ static int handle_rmode_exception(struct kvm_vcpu *vcpu,
>  	if (((vec == GP_VECTOR) || (vec == SS_VECTOR)) && err_code == 0)
>  		if (emulate_instruction(vcpu, NULL, 0, 0, 0) == EMULATE_DONE)
>  			return 1;
> +	/*
> +	 * Forward #DB
> +	 * FIXME: Quick-hack, breaks guest debugging in real mode, will be
> +	 *        fixed with the required debugging infrastructure rework.
> +	 */
> +	if (vec == 1) {
>   

DB_VECTOR

> +		vmx_inject_irq(vcpu, vec);
>   

kvm_queue_exception()

> +		return 1;
> +	}
>  	return 0;
>  }
>   
>   

But it seems the best thing to do is not to trap #DB in 
update_exception_bitmap()?

> /me now wonders if there are not even more exceptions that have to be
> forwarded. Right now we catch them all, but I did not find some path
> via which actual ones are pushed to the guest.

Looks like update_exception_bitmap() assumes none.  From a cursory look, 
seems like #DE (divide error), #DB, #BP (Breakpoint), #OF (overflow), 
#BR (range check) all need to be passed to the guest.

-- 
error compiling committee.c: too many arguments to function