linux-kernel - Re: [PATCH 01/10] iommu/ioasid: Introduce system-wide capacity

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Message-ID: <20200401134552.GD882512@myrica>
Date:   Wed, 1 Apr 2020 15:45:52 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH 01/10] iommu/ioasid: Introduce system-wide capacity

On Wed, Mar 25, 2020 at 10:55:22AM -0700, Jacob Pan wrote:
> IOASID is a limited system-wide resource that can be allocated at
> runtime. This limitation can be enumerated during boot. For example, on
> x86 platforms, PCI Process Address Space ID (PASID) allocation uses
> IOASID service. The number of supported PASID bits are enumerated by
> extended capability register as defined in the VT-d spec.
> 
> This patch adds a helper to set the system capacity, it expected to be
> set during boot prior to any allocation request.

This one-time setting is a bit awkward. Since multiple IOMMU drivers may
be loaded, this can't be a module_init() thing. And we generally have
multiple SMMU instances in the system. So we'd need to call
install_capacity() only for the first SMMU loaded with an arbitrary 1<<20,
even though each SMMU can support different numbers of PASID bits.
Furthermore, modules such as iommu-sva will want to initialize their
IOASID set at module_init(), which will happen before the SMMU can set up
the capacity, so ioasid_alloc_set() will return an error.

How about hardcoding ioasid_capacity to 20 bits for now?  It's the PCIe
limit and probably won't have to increase for a while.

Thanks,
Jean

> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>  drivers/iommu/ioasid.c | 15 +++++++++++++++
>  include/linux/ioasid.h |  5 ++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> index 0f8dd377aada..4026e52855b9 100644
> --- a/drivers/iommu/ioasid.c
> +++ b/drivers/iommu/ioasid.c
> @@ -17,6 +17,21 @@ struct ioasid_data {
>  	struct rcu_head rcu;
>  };
>  
> +static ioasid_t ioasid_capacity;
> +static ioasid_t ioasid_capacity_avail;
> +
> +/* System capacity can only be set once */
> +void ioasid_install_capacity(ioasid_t total)
> +{
> +	if (ioasid_capacity) {
> +		pr_warn("IOASID capacity already set at %d\n", ioasid_capacity);
> +		return;
> +	}
> +
> +	ioasid_capacity = ioasid_capacity_avail = total;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_install_capacity);
> +
>  /*
>   * struct ioasid_allocator_data - Internal data structure to hold information
>   * about an allocator. There are two types of allocators:
> diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> index 6f000d7a0ddc..9711fa0dc357 100644
> --- a/include/linux/ioasid.h
> +++ b/include/linux/ioasid.h
> @@ -40,7 +40,7 @@ void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
>  int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
>  void ioasid_unregister_allocator(struct ioasid_allocator_ops *allocator);
>  int ioasid_set_data(ioasid_t ioasid, void *data);
> -
> +void ioasid_install_capacity(ioasid_t total);
>  #else /* !CONFIG_IOASID */
>  static inline ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min,
>  				    ioasid_t max, void *private)
> @@ -72,5 +72,8 @@ static inline int ioasid_set_data(ioasid_t ioasid, void *data)
>  	return -ENOTSUPP;
>  }
>  
> +static inline void ioasid_install_capacity(ioasid_t total)
> +{
> +}
>  #endif /* CONFIG_IOASID */
>  #endif /* __LINUX_IOASID_H */
> -- 
> 2.7.4
>