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

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Date:   Wed, 16 Feb 2022 15:11:15 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Christoph Hellwig <hch@....de>
Cc:     baolu.lu@...ux.intel.com, 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

On 2/16/22 11:45 AM, Tian, Kevin wrote:
>> 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.

Emm, I have no idea either. Let me check it with the architecture
experts.

> 
> 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.

Best regards,
baolu