linux-kernel - Re: [PATCH v2 10/11] iommu/vt-d: Use xarray for global device_domain

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Date:   Mon, 14 Feb 2022 10:00:38 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <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>,
        Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 10/11] iommu/vt-d: Use xarray for global
 device_domain_info

On Mon, Feb 14, 2022 at 10:57:03AM +0800, Lu Baolu wrote:
> Replace the existing global device_domain_list with an array so that it
> could be rapidly searched. The index of the array is composed by the PCI
> segment, bus and devfn. Use RCU for lock protection.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>  include/linux/intel-iommu.h |  1 -
>  drivers/iommu/intel/iommu.c | 72 ++++++++++++++++++-------------------
>  2 files changed, 34 insertions(+), 39 deletions(-)
> 
> diff --git a/include/linux/intel-iommu.h b/include/linux/intel-iommu.h
> index 03f1134fc2fe..aca1c1cc04a8 100644
> +++ b/include/linux/intel-iommu.h
> @@ -610,7 +610,6 @@ struct intel_iommu {
>  /* PCI domain-device relationship */
>  struct device_domain_info {
>  	struct list_head link;	/* link to domain siblings */
> -	struct list_head global; /* link to global list */
>  	struct list_head table;	/* link to pasid table */
>  	u32 segment;		/* PCI segment number */
>  	u8 bus;			/* PCI bus number */
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index fb17ed8c08f3..ecec923ce191 100644
> +++ b/drivers/iommu/intel/iommu.c
> @@ -131,8 +131,6 @@ static struct intel_iommu **g_iommus;
>  
>  static void __init check_tylersburg_isoch(void);
>  static int rwbf_quirk;
> -static inline struct device_domain_info *
> -dmar_search_domain_by_dev_info(int segment, int bus, int devfn);
>  
>  /*
>   * set to 1 to panic kernel if can't successfully enable VT-d
> @@ -318,30 +316,34 @@ int intel_iommu_gfx_mapped;
>  EXPORT_SYMBOL_GPL(intel_iommu_gfx_mapped);
>  
>  DEFINE_SPINLOCK(device_domain_lock);
> -static LIST_HEAD(device_domain_list);
> +static DEFINE_XARRAY_ALLOC(device_domain_array);

The 'device_domain_lock' should be replaced by the internal xarray
spinlock, no reason to have two locks.

> +
> +/* Convert device source ID into the index of device_domain_array. */
> +static inline unsigned long devi_idx(unsigned long seg, u8 bus, u8 devfn)
> +{
> +	return (seg << 16) | PCI_DEVID(bus, devfn);
> +}
>  
>  /*
> - * Iterate over elements in device_domain_list and call the specified
> + * Iterate over elements in device_domain_array and call the specified
>   * callback @fn against each element.
>   */
>  int for_each_device_domain(int (*fn)(struct device_domain_info *info,
>  				     void *data), void *data)
>  {
> -	int ret = 0;
> -	unsigned long flags;
>  	struct device_domain_info *info;
> +	unsigned long index;
> +	int ret = 0;
>  
> -	spin_lock_irqsave(&device_domain_lock, flags);
> -	list_for_each_entry(info, &device_domain_list, global) {
> +	rcu_read_lock();
> +	xa_for_each(&device_domain_array, index, info) {
>  		ret = fn(info, data);
> -		if (ret) {
> -			spin_unlock_irqrestore(&device_domain_lock, flags);
> -			return ret;
> -		}
> +		if (ret)
> +			break;

And you probably shouldn't try to use RCU. It is really unclear how
this function can be useful while racing against
intel_iommu_release_device(), eg today the only user of this function
does:

static int search_pasid_table(struct device_domain_info *info, void *opaque)
{
	struct pasid_table_opaque *data = opaque;

	if (info->iommu->segment == data->segment &&
	    info->bus == data->bus &&
	    info->devfn == data->devfn &&

And even if you kfree_rcu(info) then 'info->iommu->' is still racy
unlocked.

RCU is complicated to use, it is not just a drop in replacement for a
spinlock.

Jason