netdev - Re: [Xen-devel] [RFC 21/23] net/xen-netback: Make it running on 64KB page granularity

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Date:	Wed, 20 May 2015 15:26:46 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>,
	Julien Grall <julien.grall@...rix.com>
CC:	<ian.campbell@...rix.com>, <stefano.stabellini@...citrix.com>,
	<netdev@...r.kernel.org>, <tim@....org>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [RFC 21/23] net/xen-netback: Make it running on 64KB
 page granularity

On 20/05/15 09:26, Wei Liu wrote:
> On Tue, May 19, 2015 at 11:56:39PM +0100, Julien Grall wrote:
> 
>>
>>>> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
>>>> index 0eda6e9..c2a5402 100644
>>>> --- a/drivers/net/xen-netback/common.h
>>>> +++ b/drivers/net/xen-netback/common.h
>>>> @@ -204,7 +204,7 @@ struct xenvif_queue { /* Per-queue data for xenvif */
>>>>  /* Maximum number of Rx slots a to-guest packet may use, including the
>>>>   * slot needed for GSO meta-data.
>>>>   */
>>>> -#define XEN_NETBK_RX_SLOTS_MAX (MAX_SKB_FRAGS + 1)
>>>> +#define XEN_NETBK_RX_SLOTS_MAX ((MAX_SKB_FRAGS + 1) * XEN_PFN_PER_PAGE)
>>>>
>>>>  enum state_bit_shift {
>>>>         /* This bit marks that the vif is connected */
>>>>
>>>> The function xenvif_wait_for_rx_work never returns. I guess it's because there
>>>> is not enough slot available.
>>>>
>>>> For 64KB page granularity we ask for 16 times more slots than 4KB page
>>>> granularity. Although, it's very unlikely that all the slot will be used.
>>>>
>>>> FWIW I pointed out the same problem on blkfront.
>>>>
>>>
>>> This is not going to work. The ring in netfront / netback has only 256
>>> slots. Now you ask for netback to reserve more than 256 slots -- (17 +
>>> 1) * (64 / 4) = 288, which can never be fulfilled. See the call to
>>> xenvif_rx_ring_slots_available.
>>>
>>> I think XEN_NETBK_RX_SLOTS_MAX derived from the fact the each packet to
>>> the guest cannot be larger than 64K. So you might be able to
>>>
>>> #define XEN_NETBK_RX_SLOTS_MAX ((65536 / XEN_PAGE_SIZE) + 1)
>>
>> I didn't know that packet cannot be larger than 64KB. That's simply a lot
>> the problem.
>>
> 
> I think about this more, you will need one more slot for GSO
> information, so make it ((65536 / XEN_PAGE_SIZE) + 1 + 1).

I have introduced a XEN_MAX_SKB_FRAGS (65536 / XEN_PAGE_SIZE + 1)
because it's required in another place.

Regards,

-- 
Julien Grall
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