linux-kernel - RE: [PATCH v2 03/12] iommu/vt-d: Remove clearing translation data in disable_dmar

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Message-ID: <BN9PR11MB52762E7602FFF7EE4B52AC888CAA9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Tue, 14 Jun 2022 06:49:18 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Jason Gunthorpe" <jgg@...dia.com>
CC:     Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.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 03/12] iommu/vt-d: Remove clearing translation data in
 disable_dmar_iommu()

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, June 14, 2022 10:51 AM
> 
> The disable_dmar_iommu() is called when IOMMU initialization fails or
> the IOMMU is hot-removed from the system. In both cases, there is no
> need to clear the IOMMU translation data structures for devices.
> 
> On the initialization path, the device probing only happens after the
> IOMMU is initialized successfully, hence there're no translation data
> structures.

Out of curiosity. With kexec the IOMMU may contain stale mappings
from the old kernel. Then is it meaningful to disable IOMMU after the
new kernel fails to initialize it properly?

> 
> On the hot-remove path, there is no real use case where the IOMMU is
> hot-removed, but the devices that it manages are still alive in the
> system. The translation data structures were torn down during device
> release, hence there's no need to repeat it in IOMMU hot-remove path
> either. This removes the unnecessary code and only leaves a check.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/intel/pasid.h |  1 +
>  drivers/iommu/intel/iommu.c | 21 +++++++--------------
>  2 files changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iommu/intel/pasid.h b/drivers/iommu/intel/pasid.h
> index 583ea67fc783..2afbb2afe8cc 100644
> --- a/drivers/iommu/intel/pasid.h
> +++ b/drivers/iommu/intel/pasid.h
> @@ -39,6 +39,7 @@
>   * only and pass-through transfer modes.
>   */
>  #define FLPT_DEFAULT_DID		1
> +#define NUM_RESERVED_DID		2
> 
>  /*
>   * The SUPERVISOR_MODE flag indicates a first level translation which
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index ff6018f746e0..695aed474e5d 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1715,23 +1715,16 @@ static int iommu_init_domains(struct
> intel_iommu *iommu)
> 
>  static void disable_dmar_iommu(struct intel_iommu *iommu)
>  {
> -	struct device_domain_info *info, *tmp;
> -	unsigned long flags;
> -
>  	if (!iommu->domain_ids)
>  		return;
> 
> -	spin_lock_irqsave(&device_domain_lock, flags);
> -	list_for_each_entry_safe(info, tmp, &device_domain_list, global) {
> -		if (info->iommu != iommu)
> -			continue;
> -
> -		if (!info->dev || !info->domain)
> -			continue;
> -
> -		__dmar_remove_one_dev_info(info);
> -	}
> -	spin_unlock_irqrestore(&device_domain_lock, flags);
> +	/*
> +	 * All iommu domains must have been detached from the devices,
> +	 * hence there should be no domain IDs in use.
> +	 */
> +	if (WARN_ON(bitmap_weight(iommu->domain_ids,
> cap_ndoms(iommu->cap))
> +		    != NUM_RESERVED_DID))
> +		return;
> 
>  	if (iommu->gcmd & DMA_GCMD_TE)
>  		iommu_disable_translation(iommu);
> --
> 2.25.1