linux-kernel - Re: [PATCH v3 7/7] iommu/riscv: Paging domain support

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Message-ID: <20240501145621.GD1723318@ziepe.ca>
Date: Wed, 1 May 2024 11:56:21 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Tomasz Jeznach <tjeznach@...osinc.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Anup Patel <apatel@...tanamicro.com>,
	Sunil V L <sunilvl@...tanamicro.com>,
	Nick Kossifidis <mick@....forth.gr>,
	Sebastien Boeuf <seb@...osinc.com>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	iommu@...ts.linux.dev, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux@...osinc.com
Subject: Re: [PATCH v3 7/7] iommu/riscv: Paging domain support

On Tue, Apr 30, 2024 at 01:01:57PM -0700, Tomasz Jeznach wrote:

> +#define iommu_domain_to_riscv(iommu_domain) \
> +	container_of(iommu_domain, struct riscv_iommu_domain, domain)
> +
> +#define dev_to_domain(dev) \
> +	iommu_domain_to_riscv(dev_iommu_priv_get(dev))

Please use the priv properly and put a struct around it, you'll
certainly need this eventually to do the rest of the advanced
features.

> +static void riscv_iommu_bond_unlink(struct riscv_iommu_domain *domain, struct device *dev)
> +{
> +	struct riscv_iommu_bond *bond, *found = NULL;
> +	unsigned long flags;
> +
> +	if (!domain)
> +		return;
> +
> +	spin_lock_irqsave(&domain->lock, flags);

This is never locked from an irq, you don't need to use the irqsave
variations.

> +	list_for_each_entry_rcu(bond, &domain->bonds, list) {
> +		if (bond->dev == dev) {
> +			list_del_rcu(&bond->list);
> +			found = bond;
> +		}
> +	}
> +	spin_unlock_irqrestore(&domain->lock, flags);
> +
> +	/* Release and wait for all read-rcu critical sections have completed. */
> +	kfree_rcu(found, rcu);
> +	synchronize_rcu();

Please no, synchronize_rcu() on a path like this is not so
reasonable.. Also you don't need kfree_rcu() if you write it like this.

This still looks better to do what I said before, put the iommu not
the dev in the bond struct.


> +static struct iommu_domain *riscv_iommu_alloc_paging_domain(struct device *dev)
> +{
> +	struct riscv_iommu_domain *domain;
> +	struct riscv_iommu_device *iommu;
> +
> +	iommu = dev ? dev_to_iommu(dev) : NULL;
> +	domain = kzalloc(sizeof(*domain), GFP_KERNEL);
> +	if (!domain)
> +		return ERR_PTR(-ENOMEM);
> +
> +	INIT_LIST_HEAD_RCU(&domain->bonds);
> +	spin_lock_init(&domain->lock);
> +	domain->numa_node = NUMA_NO_NODE;
> +
> +	/*
> +	 * Follow system address translation mode.
> +	 * RISC-V IOMMU ATP mode values match RISC-V CPU SATP mode values.
> +	 */
> +	domain->pgd_mode = satp_mode >> SATP_MODE_SHIFT;

This seems really strange, the iommu paging domains should be
unrelated to what the CPU is doing. There is no connection between
these two concepts.

Just pick a size that the iommu supports.

The number of radix levels is a tunable alot of iommus have that we
haven't really exposed to anything else yet.

> +	/*
> +	 * Note: RISC-V Privilege spec mandates that virtual addresses
> +	 * need to be sign-extended, so if (VA_BITS - 1) is set, all
> +	 * bits >= VA_BITS need to also be set or else we'll get a
> +	 * page fault. However the code that creates the mappings
> +	 * above us (e.g. iommu_dma_alloc_iova()) won't do that for us
> +	 * for now, so we'll end up with invalid virtual addresses
> +	 * to map. As a workaround until we get this sorted out
> +	 * limit the available virtual addresses to VA_BITS - 1.
> +	 */
> +	domain->domain.geometry.aperture_start = 0;
> +	domain->domain.geometry.aperture_end = DMA_BIT_MASK(VA_BITS - 1);
> +	domain->domain.geometry.force_aperture = true;

Yikes.. This is probably the best solution long term anyhow, unless
you need to use the last page in VFIO for some reason.
  
>  static int riscv_iommu_device_domain_type(struct device *dev)
>  {
> -	return IOMMU_DOMAIN_IDENTITY;
> +	struct riscv_iommu_device *iommu = dev_to_iommu(dev);
> +
> +	if (iommu->ddt_mode == RISCV_IOMMU_DDTP_MODE_BARE)
> +		return IOMMU_DOMAIN_IDENTITY;
> +

Is there even a point of binding an iommu driver if the HW can't
support a DDT table? Just return -ENODEV from probe_device?

Logically a iommu block that can't decode the RID has no association
at all with a Linux struct device :)

Jason