linux-kernel - Re: [PATCH v3 5/8] iommu/vt-d: Implement def_domain

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Message-ID: <20190429200338.GA8412@infradead.org>
Date:   Mon, 29 Apr 2019 13:03:38 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, kevin.tian@...el.com,
        ashok.raj@...el.com, dima@...sta.com, tmurphy@...sta.com,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        jacob.jun.pan@...el.com
Subject: Re: [PATCH v3 5/8] iommu/vt-d: Implement def_domain_type iommu ops
 entry

> @@ -3631,35 +3607,30 @@ static int iommu_no_mapping(struct device *dev)
>  	if (iommu_dummy(dev))
>  		return 1;
>  
> -	if (!iommu_identity_mapping)
> -		return 0;
> -

FYI, iommu_no_mapping has been refactored in for-next:

https://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git/commit/?h=x86/vt-d&id=48b2c937ea37a3bece0094b46450ed5267525289

>  	found = identity_mapping(dev);
>  	if (found) {
> +		/*
> +		 * If the device's dma_mask is less than the system's memory
> +		 * size then this is not a candidate for identity mapping.
> +		 */
> +		u64 dma_mask = *dev->dma_mask;
> +
> +		if (dev->coherent_dma_mask &&
> +		    dev->coherent_dma_mask < dma_mask)
> +			dma_mask = dev->coherent_dma_mask;
> +
> +		if (dma_mask < dma_get_required_mask(dev)) {

I know this is mostly existing code moved around, but it really needs
some fixing.  For one dma_get_required_mask is supposed to return the
required to not bounce mask for the given device.  E.g. for a device
behind an iommu it should always just return 32-bit.  If you really
want to check vs system memory please call dma_direct_get_required_mask
without the dma_ops indirection.

Second I don't even think we need to check the coherent_dma_mask,
dma_direct is pretty good at always finding memory even without
an iommu.

Third this doesn't take take the bus_dma_mask into account.

This probably should just be:

		if (min(*dev->dma_mask, dev->bus_dma_mask) <
		    dma_direct_get_required_mask(dev)) {