netdev - Re: [PATCH v2] net: mlx5: Use iowriteXbe() to ring doorbell and remove reduntant wmb()

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Message-ID: <20200103191749.GE9706@ziepe.ca>
Date:   Fri, 3 Jan 2020 15:17:49 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     saeedm@...lanox.com, leon@...nel.org, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, eli@...lanox.com, tariqt@...lanox.com,
        danielm@...lanox.com,
        Håkon Bugge <haakon.bugge@...cle.com>
Subject: Re: [PATCH v2] net: mlx5: Use iowriteXbe() to ring doorbell and
 remove reduntant wmb()

On Fri, Jan 03, 2020 at 07:52:07PM +0200, Liran Alon wrote:
> diff --git a/include/linux/mlx5/cq.h b/include/linux/mlx5/cq.h
> index 40748fc1b11b..4631ad35da53 100644
> +++ b/include/linux/mlx5/cq.h
> @@ -162,13 +162,8 @@ static inline void mlx5_cq_arm(struct mlx5_core_cq *cq, u32 cmd,
>  
>  	*cq->arm_db = cpu_to_be32(sn << 28 | cmd | ci);
>  
> -	/* Make sure that the doorbell record in host memory is
> -	 * written before ringing the doorbell via PCI MMIO.
> -	 */
> -	wmb();
> -
> -	doorbell[0] = cpu_to_be32(sn << 28 | cmd | ci);
> -	doorbell[1] = cpu_to_be32(cq->cqn);
> +	doorbell[0] = sn << 28 | cmd | ci;
> +	doorbell[1] = cq->cqn;

This does actually have to change to a u64 otherwise it is not the
same.

On x86 LE, it was
 db[0] = swab(a)
 db[1] = swab(b)
 __raw_writel(db)

Now it is
 db[0] = a
 db[1] = b
 __raw_writel(swab(db))

Putting the swab around the u64 swaps the order of a/b in the TLP.

It might be tempting to swap db[0]/db[1] but IIRC this messed it up on
BE.

The sanest, simplest solution is to use a u64 natively, as the example
I gave did.

There is also the issue of casting a u32 to a u64 and possibly
triggering a unaligned kernel access, presumably this doesn't happen
today only by some lucky chance..

>  	mlx5_write64(doorbell, uar_page + MLX5_CQ_DOORBELL);
>  }
> diff --git a/include/linux/mlx5/doorbell.h b/include/linux/mlx5/doorbell.h
> index 5c267707e1df..9c1d35777323 100644
> +++ b/include/linux/mlx5/doorbell.h
> @@ -43,17 +43,15 @@
>   * Note that the write is not atomic on 32-bit systems! In contrast to 64-bit
>   * ones, it requires proper locking. mlx5_write64 doesn't do any locking, so use
>   * it at your own discretion, protected by some kind of lock on 32 bits.
> - *
> - * TODO: use write{q,l}_relaxed()
>   */
>  
> -static inline void mlx5_write64(__be32 val[2], void __iomem *dest)
> +static inline void mlx5_write64(u32 val[2], void __iomem *dest)
>  {

So this should accept a straight u64, the goofy arrays have to go away

>  #if BITS_PER_LONG == 64
> -	__raw_writeq(*(u64 *)val, dest);
> +	iowrite64be(*(u64 *)val, dest);
>  #else
> -	__raw_writel((__force u32) val[0], dest);
> -	__raw_writel((__force u32) val[1], dest + 4);
> +	iowrite32be(val[0], dest);
> +	iowrite32be(val[1], dest + 4);

With a u64 input this fallback is written as

  iowrite32be(val >> 32, dest)
  iowrite32be((u32)val, dest + 4)

Which matches the definition for how write64 must construct a TLP.

And arguably the first one should be _relaxed (but nobody cares about
this code path)

Jason