On 11 August 2014 02:48, Wei Hu <w...@microsoft.com> wrote:
> CC freebsd-net@ for wider discussion.
>
> Hi Adrian,
>
> Many thanks for the explanation. I checked the if_igb.c and found the
> flowid field was set in the RX side in igb_rxeof():
>
> Igb_rxeof()
> {
> ...
> #ifdef RSS
> /* XXX set flowtype once this works right */
> rxr->fmp->m_pkthdr.flowid =
> le32toh(cur->wb.lower.hi_dword.rss);
> rxr->fmp->m_flags |= M_FLOWID;
> ...
> }
>
> I have two questions regarding this.
>
> 1. Is the RSS hash value stored in cur->wb.lower.hi_dword.rss set by the NIC
> hardware?
Yup.
> 2. So the hash value and m_flags are stored in the mbuf related to the
> received packet on the rx side(lgb_rxeof()). But we check the hash value and
> m_flags in mbuf related to the send packet on the tx side (in
> igb_mq_start()). Does the kernel re-use the same mbuf for tx? If so, how does
> it know for the same network stream it should use the same mbuf got from the
> rx for packet sending? If not, how does the kernel preserve the same hash
> value across the rx mbuf and tx mbuf for same network stream? This seems
> quite magical to me.
The mbuf flowid/flowtype ends up in the inpcb->inp_flowid /
inpcb->inp_flowtype as part of the TCP receive path.
Then whenever the TCP code outputs an mbuf, it copies the inpcb flow
details out to outbound mbufs.
>
> For the Hyper-V case, the host controls which vCPU it wants to interrupt. And
> the rule can change dynamically based on the load. For a non-busy VM, host
> will send most packets to same vCPU for power saving purpose. For a busy VM,
> host will distribute the packets evenly across all vCPUs. This means host
> could change the RSS bucket mapping dynamically. Hyper-V does this by sending
> a mapping table to VM whenever the it needs update. This also means we cannot
> use FreeBSD's own bucket mapping which I believe is fixed. Also Hyper-V use
> its own hash key. So do you think it is possible we still use the exisiting
> RSS infrastructure built in FreeBSD in this purpose?
Eventually. Doing rebalancing in RSS is on the TODO list, after I get
the rest of the basic packet handling / routing done.
How's vRSS notify the VM that the mapping table has changed? What's
the format of it look like?
-a
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