linux-kernel - Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

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Message-ID: <Y+pG9kWzR2c5I9FU@nvidia.com>
Date:   Mon, 13 Feb 2023 10:19:34 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     iommu@...ts.linux.dev, Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
> The iommu_group_store_type() requires the devices in the iommu group are
> not bound to any device driver during the whole operation. The existing
> code locks the device with device_lock(dev) and use device_is_bound() to
> check whether any driver is bound to device.
> 
> In fact, this can be achieved through the DMA ownership helpers. Replace
> them with iommu_group_claim/release_dma_owner() helpers.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/iommu.c | 27 +++++++++++++--------------
>  1 file changed, 13 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 4f71dcd2621b..6547cb38480c 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2807,12 +2807,6 @@ static int iommu_change_dev_def_domain(struct iommu_group *group,
>  
>  	mutex_lock(&group->mutex);
>  
> -	if (group->default_domain != group->domain) {
> -		dev_err_ratelimited(prev_dev, "Group not assigned to default domain\n");
> -		ret = -EBUSY;
> -		goto out;
> -	}
> -
>  	/*
>  	 * iommu group wasn't locked while acquiring device lock in
>  	 * iommu_group_store_type(). So, make sure that the device count hasn't
> @@ -2971,6 +2965,7 @@ static void iommu_group_unfreeze_dev_ops(struct iommu_group *group)
>  static ssize_t iommu_group_store_type(struct iommu_group *group,
>  				      const char *buf, size_t count)
>  {
> +	bool group_owner_claimed = false;
>  	struct group_device *grp_dev;
>  	struct device *dev;
>  	int ret, req_type;
> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct iommu_group *group,
>  	else
>  		return -EINVAL;
>  
> +	if (req_type != IOMMU_DOMAIN_DMA_FQ ||
> +	    group->default_domain->type != IOMMU_DOMAIN_DMA) {
> +		ret = iommu_group_claim_dma_owner(group, (void *)buf);
> +		if (ret)
> +			return ret;
> +		group_owner_claimed = true;
> +	}

I don't get it, this should be done unconditionally. If we couldn't
take ownership then we simply can't progress.

But there is more to it than that, a device that is owned should not
be release and to achieve this the general logic around the owner
scheme assumes that a driver is attached.

So if you call it from this non-driver context you have to hold the
group_mutex as previously discussed, which also means this needs to be
an externally version of iommu_group_claim_dma_owner()

Jason