linux-kernel - Re: [PATCH 3/7] iommu/vt-d: Enhance compatibility check for paging domain attach

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Message-ID: <20241011162703.GL762027@ziepe.ca>
Date: Fri, 11 Oct 2024 13:27:03 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: iommu@...ts.linux.dev, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
	Vasant Hegde <vasant.hegde@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] iommu/vt-d: Enhance compatibility check for paging
 domain attach

On Fri, Oct 11, 2024 at 12:27:18PM +0800, Lu Baolu wrote:

> @@ -1623,27 +1623,15 @@ static int domain_context_mapping_one(struct dmar_domain *domain,
>  
>  	copied_context_tear_down(iommu, context, bus, devfn);
>  	context_clear_entry(context);
> -
>  	context_set_domain_id(context, did);
>  
> -	/*
> -	 * Skip top levels of page tables for iommu which has
> -	 * less agaw than default. Unnecessary for PT mode.
> -	 */
> -	for (agaw = domain->agaw; agaw > iommu->agaw; agaw--) {
> -		ret = -ENOMEM;
> -		pgd = phys_to_virt(dma_pte_addr(pgd));
> -		if (!dma_pte_present(pgd))
> -			goto out_unlock;
> -	}

Yikes, this is nasty racy stuff, glad to see it go

But should the agaw stuff be in its own patch?

> @@ -3506,27 +3483,26 @@ int prepare_domain_attach_device(struct iommu_domain *domain,
>  	if (domain->dirty_ops && !ssads_supported(iommu))
>  		return -EINVAL;
>  
> -	/* check if this iommu agaw is sufficient for max mapped address */
> -	addr_width = agaw_to_width(iommu->agaw);
> -	if (addr_width > cap_mgaw(iommu->cap))
> -		addr_width = cap_mgaw(iommu->cap);
> -
> -	if (dmar_domain->max_addr > (1LL << addr_width))
> +	if (dmar_domain->iommu_coherency !=
> +			iommu_paging_structure_coherency(iommu))
>  		return -EINVAL;
> -	dmar_domain->gaw = addr_width;
> -
> -	/*
> -	 * Knock out extra levels of page tables if necessary
> -	 */
> -	while (iommu->agaw < dmar_domain->agaw) {
> -		struct dma_pte *pte;
> -
> -		pte = dmar_domain->pgd;
> -		if (dma_pte_present(pte)) {
> -			dmar_domain->pgd = phys_to_virt(dma_pte_addr(pte));
> -			iommu_free_page(pte);
> -		}
> -		dmar_domain->agaw--;
> +
> +	if (domain->type & __IOMMU_DOMAIN_PAGING) {

It looks like this entire function is already never called for
anything but paging?

The only three callers are:

	.default_domain_ops = &(const struct iommu_domain_ops) {
		.attach_dev		= intel_iommu_attach_device,
		.set_dev_pasid		= intel_iommu_set_dev_pasid,

and

static const struct iommu_domain_ops intel_nested_domain_ops = {
	.attach_dev		= intel_nested_attach_dev,

And none of those cases can be anything except a paging domain by
definition.

So this if should go away, or be turned into a WARN_ON.

Jason