Re: [PATCH 3/4] xen/hvc: Check HVM_PARAM_CONSOLE_[EVTCHN|PFN] for correctness.

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 23 May 2012, Konrad Rzeszutek Wilk wrote:
> We need to make sure that those parameters are setup to be correct.
> As such the value of 0 is deemed invalid and we find that we
> bail out. The hypervisor sets by default all of them to be zero
> and when the hypercall is done does a simple:
> 
>  a.value = d->arch.hvm_domain.params[a.index];
> 
> Which means that if the Xen toolstack forgot to setup the proper
> HVM_PARAM_CONSOLE_EVTCHN, we would get the default value of 0
> and use that.

I see, so the default value for v needs to be 0, because it's what the
hypervisor would return anyway if the parameter hasn't been set.


> CC: stable@kernel.org
> Fixes-Oracle-Bug: 14091238
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
>  drivers/tty/hvc/hvc_xen.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 3277f0e..5fb1cb9 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -216,12 +216,12 @@ static int xen_hvm_console_init(void)
>  		return 0;
>  
>  	r = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, &v);
> -	if (r < 0)
> +	if (r < 0 || v == 0)
>  		goto err;
>  	info->evtchn = v;
>  	v = 0;
>  	r = hvm_get_parameter(HVM_PARAM_CONSOLE_PFN, &v);
> -	if (r < 0)
> +	if (r < 0 || v == 0)
>  		goto err;
>  	mfn = v;
>  	info->intf = ioremap(mfn << PAGE_SHIFT, PAGE_SIZE);

I think that we should add a comment stating that even though mfn = 0
and evtchn = 0 are theoretically correct values, in practice they never
are and they mean that a legacy toolstack hasn't initialized the pv
console correctly.