Re: [PATCH net-next v3 2/9] xen-netback: Change TX path from grant copy to mapping

[Zoltan Kiss <zoltan.kiss@citrix.com>]

On 10/01/14 11:45, Wei Liu wrote:
> On Fri, Jan 10, 2014 at 11:35:08AM +0000, Zoltan Kiss wrote:
> [...]
>>
>>>> @@ -920,6 +852,18 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
>>>>   	err = gop->status;
>>>>   	if (unlikely(err))
>>>>   		xenvif_idx_release(vif, pending_idx, XEN_NETIF_RSP_ERROR);
>>>> +	else {
>>>> +		if (vif->grant_tx_handle[pending_idx] !=
>>>> +			NETBACK_INVALID_HANDLE) {
>>>> +			netdev_err(vif->dev,
>>>> +				"Stale mapped handle! pending_idx %x handle %x\n",
>>>> +				pending_idx, vif->grant_tx_handle[pending_idx]);
>>>> +			BUG();
>>>> +		}
>>>> +		set_phys_to_machine(idx_to_pfn(vif, pending_idx),
>>>> +			FOREIGN_FRAME(gop->dev_bus_addr >> PAGE_SHIFT));
>>>
>>> What happens when you don't have this?
>> Your frags will be filled with garbage. I don't understand exactly
>> what this function does, someone might want to enlighten us? I've
>> took it's usage from classic kernel.
>> Also, it might be worthwhile to check the return value and BUG if
>> it's false, but I don't know what exactly that return value means.
>>
>
> This is actually part of gnttab_map_refs. As you're using hypercall
> directly this becomes very fragile.
>
> So the right thing to do is to fix gnttab_map_refs.
I agree, as I mentioned in other email in this thread, I think that 
should be the topic of an another patchseries. In the meantime, I will 
use gnttab_batch_map instead of the direct hypercall, it handles the 
GNTST_eagain scenario, and I will use set_phys_to_machine the same way 
as m2p_override does:

if (unlikely(!set_phys_to_machine(idx_to_pfn(vif, pending_idx), 
FOREIGN_FRAME(gop->dev_bus_addr >> PAGE_SHIFT))
			BUG();

Zoli