linux-kernel - Re: [PATCH v1 4/6] iommu/vt-d: Add VTD_FLAG_IOMMU

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Date:   Fri, 1 Jul 2022 11:13:07 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Steve Wahl <steve.wahl@....com>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        "Anderson, Russ" <russ.anderson@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 4/6] iommu/vt-d: Add VTD_FLAG_IOMMU_PROBED flag

On 6/30/22 4:29 PM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Saturday, June 25, 2022 8:52 PM
>>
>> In the IOMMU hot-add path, there's a need to check whether an IOMMU
>> has been probed. Instead of checking the IOMMU pointer in the global
>> list, it's better to allocate a flag bit in iommu->flags for this
>> purpose.
> 
> Sorry I didn't get the point of original check. This is the hotplug path
> hence the caller of this function should already figure out it's a new
> iommu before reaching this point?
> 

Either did I. It was added by below commit without any comments about
this check.

commit ffebeb46dd34736c90ffbca1ccb0bef8f4827c44
Author: Jiang Liu <jiang.liu@...ux.intel.com>
Date:   Sun Nov 9 22:48:02 2014 +0800

     iommu/vt-d: Enhance intel-iommu driver to support DMAR unit hotplug

     Implement required callback functions for intel-iommu driver
     to support DMAR unit hotplug.

     Signed-off-by: Jiang Liu <jiang.liu@...ux.intel.com>
     Reviewed-by: Yijing Wang <wangyijing@...wei.com>
     Signed-off-by: Joerg Roedel <jroedel@...e.de>

I went through the whole hot-add process and found this check seemed to
be duplicate.

Hot-add process starts from dmar_device_hotplug(), it uses a rwlock to
synchronize the hot-add paths.

2386         down_write(&dmar_global_lock);
2387         if (insert)
2388                 ret = dmar_hotplug_insert(tmp);
2389         else
2390                 ret = dmar_hotplug_remove(tmp);
2391         up_write(&dmar_global_lock);

dmar_device_hotplug()
->dmar_hotplug_insert()
-->dmar_parse_one_drhd()   /* the added intel_iommu is allocated here*/
-->dmar_hp_add_drhd()	   /* the intel_iommu is about to bring up */
--->intel_iommu_add()

The duplicate check here:

         if (g_iommus[iommu->seq_id])
                 return 0;

All the iommu units are allocated and then initialized in the same
synchronized path. There is no need to check a duplicate initialization.

I would like to remove this check if no objection.

Best regards,
baolu