Subject: Re: [PATCH net-next v6 4/7] ixgbevf: Add a RETA query code

From: Alexander Duyck <[email-masked]>
Date: Thu, 26 Mar 2015 11:25:29 -0700

On 03/26/2015 11:10 AM, Vlad Zolotarov wrote:
>
>
> On 03/25/15 23:04, Alexander Duyck wrote:
>>
>> On 03/25/2015 01:17 PM, Vlad Zolotarov wrote:
>>>
>>>
>>> On 03/25/15 20:35, Tantilov, Emil S wrote:
>>>>> -----Original Message-----
>>>>> From: Vlad Zolotarov [mailto:[email-masked]]
>>>>> Sent: Wednesday, March 25, 2015 2:28 AM
>>>>> Subject: Re: [PATCH net-next v6 4/7] ixgbevf: Add a RETA query code
>>>> <snip>
>>>>
>>>>>> Have you tested what happens if you run:
>>>>>>
>>>>>> while true
>>>>>> do
>>>>>>     ethtool --show-rxfh-indir ethX
>>>>>> done
>>>>>>
>>>>>> in the background while passing traffic through the VF?
>>>>> I understand your concerns but let's start with clarifying a few 
>>>>> things.
>>>>> First, VF driver is by definition not trusted. If it (or its user)
>>>>> decides to do anything malicious (like u proposed above) that would
>>>>> eventually hurt (only this) VF's performance - nobody should care.
>>>>> However the right question here would be: "How the above use case may
>>>>> hurt the corresponding PF or other VFs' performance?" And since the
>>>>> mailbox operation involves quite a few MMIO writes and reads this may
>>>>> slow the PF quite a bit and this may be a problem that should be 
>>>>> taken
>>>>> care of. However it wasn't my patch series that have introduced 
>>>>> it. The
>>>>> same problem would arise if Guest would change VF's MAC address in a
>>>>> tight loop like above. Namely any VF slow path operation that would
>>>>> eventually cause the VF-PF channel transaction may be used to 
>>>>> create an
>>>>> attack on a PF.
>>>> There are operations that can be disruptive to the VF I am not 
>>>> arguing that,
>>>> the issue introduced by these patches has mostly to do with the 
>>>> fact that now
>>>> we can hit the mailbox more often for what is mostly static 
>>>> information.
>>>>
>>>> Especially with ethtool we already had to deal with an issue caused 
>>>> by net-snmp:
>>>> https://sourceforge.net/p/e1000/mailman/message/32188362/
>>>>
>>>> Where net-snmp was being too aggressive when collecting 
>>>> information, even if most of it was static.
>>>
>>> Emil, I don't really understand what are u trying to protect here 
>>> against. If a user would want to shoot him/herself in the leg - 
>>> he/she would still be able to do it with the other mailbox involving 
>>> operations like MAC change. So, what's the sense to add useless lines?
>>>
>>>>
>>>>> Perhaps storing the RSS key and the table is better option than 
>>>>> having to invoke the mailbox on every read.
>>>>> I don't think this could work if I understand your proposal 
>>>>> correctly.
>>>>> The only way to cache the result that would decrease the number of 
>>>>> mbox
>>>>> transactions would be to cache it in the VF. But how could i 
>>>>> invalidate
>>>>> this cache if the table content has been changed by a PF? I think the
>>>>> main source of a confusion here is that u assume that PF driver is a
>>>>> Linux ixgbe driver that doesn't support an indirection table 
>>>>> change at
>>>>> the moment. As I have explained above - this should not be assumed.
>>>> You keep mentioning other drivers - what other driver do you mean?
>>>> All the PF drivers that enable SRIOV are maintained and supported 
>>>> by Intel.
>>>>
>>>> For HW older than X550 we can simply not allow the RSS hash to be 
>>>> modified if the driver is loaded in SRIOV mode.
>>>> This way the RSS info can be read once the driver is loaded. For 
>>>> X550 this can all be done in the VF, so you can avoid calling the 
>>>> mailbox altogether.
>>>> I understand this is a bit limiting, but this is due to HW 
>>>> limitation anyway (VFs do not have their own RSS config).
>>>
>>> Let me remind u that Linux, FreeBSD, XEN  and DPDK PF drivers are 
>>> all open source so u can't actually go and "not allow" things. ;) 
>>> And although Intel developers contribute most of the code there are 
>>> and will be other contributors too so I doubt the proposed above 
>>> approach fits the open source spirit well. ;)
>>
>> Actually these drivers already support multiple OSes just fine. The 
>> part where I think you are confused is that you assume they all use 
>> the same Mailbox API which they likely wouldn't.  I would suggest 
>> taking a look at ixgbe_pfvf_api_rev in mbx.h of the VF driver. 
>> Different OSes have different things that can be supported, so for 
>> example the ixgbe_mbox_api_20 is reserved for a Solaris based PF/VF 
>> combination.  I would suspect that FreeBSD will likely have to 
>> conform to the existing APIs, or report that it only supports a 
>> different version of the mailbox API.
>>
>>>
>>> The user should actually not query the indirection table and a hash 
>>> key too often. And if he/she does - it should be his/her problem.
>>> However, if like with the ixgbevf_set_num_queues() u insist on your 
>>> way of doing this (on caching the indirection table and hash key) - 
>>> then please let me know and I will add it. Because, frankly, I care 
>>> about the PF part of this series much more than for the VF part... ;)
>>
>> I would say you don't need to cache it, but for 82599 and x540 there 
>> isn't any need to store more than 3 bits per entry, 384b, or 12 
>> DWORDs for the entire RETA of the VF since the hardware can support 
>> at most 8 queues w/ SR-IOV.
>
> Isn't it 2 bits? VF may have up to 8 queues in total but it supports 
> up to 4 RSS queues (according to MRQC.MRQE and PSRTYPE[n].RQPL 
> description...

You might be right.  I was thinking that there was a mode with 16 pools, 
each with 8 RSS queues per pool, however it looks like that mode is 
Virtualization w/ DCB only.

- Alex