Re: [Xen-devel] [PATCH] xen-netfront: correct MAX_TX_TARGET calculation.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.02.12 at 13:59, Laszlo Ersek <lersek@xxxxxxxxxx> wrote:
> On 02/03/12 13:27, Laszlo Ersek wrote:
>> On 01/27/12 11:36, Wei Liu wrote:
> 
>>> As the tx structure is bigger than rx structure. I think scratch space
>>> size is likely to shrink after correction.
>>
>> It also seems to affect the netfront_tx_slot_available() function,
>> making it stricter (likely). Before the patch, the function may have
>> reported available slots when there were none, causing spurious(?) queue
>> wakeups in xennet_maybe_wake_tx(), and not stopping the queue in
>> xennet_start_xmit() when it should have(?).
> 
> (Eyeballing the source makes me think
> 
>    NET_TX_RING_SIZE == (4096 - 16 - 48) / (5 * 4) == 201
>    NET_RX_RING_SIZE == (4096 - 16 - 48) / (4 * 4) == 252

NET_TX_RING_SIZE == (4096 - 16 - 48) / (6 * 2) == 336
NET_RX_RING_SIZE == (4096 - 16 - 48) / (4 * 2) == 504

and with {R,T}X_MAX_TARGET capped to 256 the change really is
benign without multi-page ring support afaict.

Jan

> but I didn't try to verify them.)
> 
> Laszlo
> 
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