Re: [PATCH v2 07/10] nvme-pci: Use PCI p2pmem subsystem to manage the CMB

[Logan Gunthorpe <logang@deltatee.com>]

On 05/03/18 12:57 PM, Sagi Grimberg wrote:
> Keith, while we're on this, regardless of cmb, is SQE memcopy and DB 
> update ordering always guaranteed?
> 
> If you look at mlx4 (rdma device driver) that works exactly the same as
> nvme you will find:
> -- 
>                  qp->sq.head += nreq;
> 
>                  /*
>                   * Make sure that descriptors are written before
>                   * doorbell record.
>                   */
>                  wmb();
> 
>                  writel(qp->doorbell_qpn,
>                         to_mdev(ibqp->device)->uar_map + 
> MLX4_SEND_DOORBELL);
> 
>                  /*
>                   * Make sure doorbells don't leak out of SQ spinlock
>                   * and reach the HCA out of order.
>                   */
>                  mmiowb();
> -- 

To me, it looks like the wmb() is redundant as writel should guarantee 
the order. (Indeed, per Sinan's comment, writel on arm64 starts with a 
wmb() which means, on that platform, there are two wmb() calls in a row.)

The mmiowb() call, on the other hand, looks correct per my understanding 
of it's purpose with respect to the spinlock.

Logan


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