netdev - Re: [PATCH rdma-next 0/4] Allow relaxed ordering read in VFs and VMs

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Message-ID: <7c5eb785-0fe7-e0e5-8232-403e1d3538ac@intel.com>
Date:   Tue, 11 Apr 2023 16:21:09 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>, Leon Romanovsky <leon@...nel.org>
CC:     Leon Romanovsky <leonro@...dia.com>,
        Avihai Horon <avihaih@...dia.com>, Aya Levin <ayal@...dia.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "Jakub Kicinski" <kuba@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, Meir Lichtinger <meirl@...lanox.com>,
        "Michael Guralnik" <michaelgur@...lanox.com>,
        <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Shay Drory <shayd@...dia.com>
Subject: Re: [PATCH rdma-next 0/4] Allow relaxed ordering read in VFs and VMs



On 4/11/2023 7:01 AM, Jason Gunthorpe wrote:
> On Mon, Apr 10, 2023 at 04:07:49PM +0300, Leon Romanovsky wrote:
>> From: Leon Romanovsky <leonro@...dia.com>
>>
>> From Avihai,
>>
>> Currently, Relaxed Ordering (RO) can't be used in VFs directly and in
>> VFs assigned to QEMU, even if the PF supports RO. This is due to issues
>> in reporting/emulation of PCI config space RO bit and due to current
>> HCA capability behavior.
>>
>> This series fixes it by using a new HCA capability and by relying on FW
>> to do the "right thing" according to the PF's PCI config space RO value.
>>
>> Allowing RO in VFs and VMs is valuable since it can greatly improve
>> performance on some setups. For example, testing throughput of a VF on
>> an AMD EPYC 7763 and ConnectX-6 Dx setup showed roughly 60% performance
>> improvement.
>>
>> Thanks
>>
>> Avihai Horon (4):
>>   RDMA/mlx5: Remove pcie_relaxed_ordering_enabled() check for RO write
>>   RDMA/mlx5: Check pcie_relaxed_ordering_enabled() in UMR
>>   net/mlx5: Update relaxed ordering read HCA capabilities
>>   RDMA/mlx5: Allow relaxed ordering read in VFs and VMs
> 
> This looks OK, but the patch structure is pretty confusing.
> 
> It seems to me there are really only two patches here, the first is to
> add some static inline
> 
> 'mlx5 supports read ro'
> 
> which supports both the cap bits described in
> the PRM, with a little comment to explain that old devices only set
> the old cap.
> 
> And a second patch to call it in all the places we need to check before
> setting the mkc ro read bit.
> 
> Maybe a final third patch to sort out that mistake in the write side.
> 
> But this really doesn't have anything to do with VFs and VMs, this is
> adjusting the code to follow the current PRM because the old one was
> mis-desgined.
> 
> Jason

FWIW I think Jason's outline here makes sense too and might be slightly
better. However, reading through the series I was reasonably able to
understand things enough that I think its fine as-is.

In some sense its not about VF or VM, but fixing this has the result
that it fixes a setup with VF and VM, so I think thats an ok thing to
call out as the goal.