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

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Date:	Fri, 10 Jan 2014 16:02:20 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<jonathan.davies@...rix.com>, <ian.campbell@...rix.com>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH net-next v3 2/9] xen-netback: Change TX path
 from grant copy to mapping

On 10/01/14 15:24, Zoltan Kiss wrote:
> 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 the grant table code doesn't provide the API calls you need you can
either:

a) add the new API as a prerequisite patch.
b) use the existing API calls and live with the performance problem,
until you can refactor the API later on.

Adding a netback-specific hack isn't a valid option.

David
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