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

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Date:   Sat, 4 Jan 2020 00:28:36 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 3 Jan 2020, at 21:17, Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> 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.

Oops. You are right...

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

I agree.

> 
> 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

I agree.

> 
>> #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)

I agree with everything. Will fix on v3.

Thanks!
-Liran

> 
> Jason