linux-kernel - RE: [PATCH v2 09/11] iommu/vt-d: Remove commented code

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Message-ID: <BN9PR11MB527698691A1DF10EEB514AA08C359@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 16 Feb 2022 03:45:33 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Christoph Hellwig <hch@....de>, Lu Baolu <baolu.lu@...ux.intel.com>
CC:     Joerg Roedel <joro@...tes.org>, "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Robin Murphy" <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 09/11] iommu/vt-d: Remove commented code

> From: Christoph Hellwig <hch@....de>
> Sent: Monday, February 14, 2022 3:34 PM
> 
> On Mon, Feb 14, 2022 at 10:57:02AM +0800, Lu Baolu wrote:
> > This removes unnecessary commented code.
> 
> Removing dead code is always good:
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> 
> But someone might really want to take a look if draining makes sense here
> or not.

Looks that dead code has been there since intel-iommu driver was
firstly introduced in 2007. I don't know whether we can dig out
the reason (Baolu, can you have a check?) why it's only special
cased for read draining but not write draining. I cannot find any
such recommendation from VT-d spec.

Looking at VT-d spec it stated that since VT-d major version 2
drain is conducted automatically by hardware and above flags
are essentially ignored. 

Given that possibly a safer option is to always set read/write
draining flags before version 2 and skip it after.

Thanks
Kevin