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SubjectRe: [PATCH v7 2/3] PCI: Enable PCIe Relaxed Ordering if supported

On 2017/7/28 1:44, Casey Leedom wrote:
> | From: Ding Tianhong <>
> | Sent: Wednesday, July 26, 2017 6:01 PM
> |
> | On 2017/7/27 3:05, Casey Leedom wrote:
> | >
> | > Ding, send me a note if you'd like me to work that [cxgb4vf patch] up
> | > for you.
> |
> | Ok, you could send the change log and I could put it in the v8 version
> | together, will you base on the patch 3/3 or build a independence patch?
> Which ever you'd prefer. It would basically mirror the same exact code that
> you've got for cxgb4. I.e. testing the setting of the VF's PCIe Capability
> Device Control[Relaxed Ordering Enable], setting a new flag in
> adpater->flags, testing that flag in cxgb4vf/sge.c:t4vf_sge_alloc_rxq().
> But since the VF's PF will already have disabled the PF's Relaxed Ordering
> Enable, the VF will also have it's Relaxed Ordering Enable disabled and any
> effort by the internal chip to send TLPs with the Relaxed Ordering Attribute
> will be gated by the PCIe logic. So it's not critical that this be in the
> first patch. Your call. Let me know if you'd like me to send that to you.

Good, please Send it to me, I will put it together and send the v8 this week,
I think Bjorn will be back next week .:)

> | From: Ding Tianhong <>
> | Sent: Wednesday, July 26, 2017 6:08 PM
> |
> | On 2017/7/27 2:26, Casey Leedom wrote:
> | >
> | > 1. Did we ever get any acknowledgement from either Intel or AMD
> | > on this patch? I know that we can't ensure that, but it sure would
> | > be nice since the PCI Quirks that we're putting in affect their
> | > products.
> |
> | Still no Intel and AMD guys has ack this, this is what I am worried about,
> | should I ping some man again ?
> By amusing coincidence, Patrik Cramer (now Cc'ed) from Intel sent me a note
> yesterday with a link to the official Intel performance tuning documentation
> which covers this issue:
> In section 3.9.1 we have:
> 3.9.1 Optimizing PCIe Performance for Accesses Toward Coherent Memory
> and Toward MMIO Regions (P2P)
> In order to maximize performance for PCIe devices in the processors
> listed in Table 3-6 below, the soft- ware should determine whether the
> accesses are toward coherent memory (system memory) or toward MMIO
> regions (P2P access to other devices). If the access is toward MMIO
> region, then software can command HW to set the RO bit in the TLP
> header, as this would allow hardware to achieve maximum throughput for
> these types of accesses. For accesses toward coherent memory, software
> can command HW to clear the RO bit in the TLP header (no RO), as this
> would allow hardware to achieve maximum throughput for these types of
> accesses.
> Table 3-6. Intel Processor CPU RP Device IDs for Processors Optimizing
> PCIe Performance
> Processor CPU RP Device IDs
> Intel Xeon processors based on 6F01H-6F0EH
> Broadwell microarchitecture
> Intel Xeon processors based on 2F01H-2F0EH
> Haswell microarchitecture
> Unfortunately that's a pretty thin section. But it does expand the set of
> Intel Root Complexes for which our Linux PCI Quirk will need to cover. So
> you should add those to the next (and hopefully final) spin of your patch.
> And, it also verifies the need to handle the use of Relaxed Ordering more
> subtlely than simply turning it off since the NVMe peer-to-peer example I
> keep bringing up would fall into the "need to use Relaxed Ordering" case ...
> It would have been nice to know why this is happening and if any future
> processor would fix this. After all, Relaxed Ordering, is just supposed to
> be a hint. At worst, a receiving device could just ignore the attribute
> entirely. Obviously someone made an effort to implement it but ... it
> didn't go the way they wanted.
> And, it also would have been nice to know if there was any hidden register
> in these Intel Root Complexes which can completely turn off the effort to
> pay attention to the Relaxed Ordering Attribute. We've spend an enormous
> amount of effort on this issue here on the Linux PCI email list struggling
> mightily to come up with a way to determine when it's
> safe/recommended/not-recommended/unsafe to use Relaxed Ordering when
> directing TLPs towards the Root Complex. And some architectures require RO
> for decent performance so we can't just "turn it off" unilatterally.

I am glad to hear that more person were focus on this problem, It would be great
if they could enter our discussion and give us more suggestion. :)


> Casey
> .

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