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

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Date:   Fri, 3 Jan 2020 15:06:39 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Will Deacon <will@...nel.org>, 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] net: mlx5: Use writeX() to ring doorbell and remove
 reduntant wmb()

On Fri, Jan 03, 2020 at 08:38:51PM +0200, Liran Alon wrote:

> Actually after re-reading AMD Optimization Guide SDM, I see it is
> guaranteed that: “Write-combining is closed if all 64 bytes of the
> write buffer are valid”.  And this is indeed always the case for AWS
> ENA LLQ. Because as can be seen at ena_com_config_llq_info(),
> desc_list_entry_size is either 128, 192 or 256. i.e. Always a
> multiple of 64 bytes. So this explains why this wasn’t an issue in
> production.

That makes a lot of sense

So, the problem with defining WC in the kernel memory model still
remains, but it is back to a theoretical one.

Jason