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Date:   Wed, 22 Sep 2021 14:45:09 +0100
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Liu Yi L <yi.l.liu@intel.com>
Cc:     alex.williamson@redhat.com, jgg@nvidia.com, hch@lst.de,
        jasowang@redhat.com, joro@8bytes.org, kevin.tian@intel.com,
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        lushenming@huawei.com, eric.auger@redhat.com, corbet@lwn.net,
        ashok.raj@intel.com, yi.l.liu@linux.intel.com,
        jun.j.tian@intel.com, hao.wu@intel.com, dave.jiang@intel.com,
        jacob.jun.pan@linux.intel.com, kwankhede@nvidia.com,
        robin.murphy@arm.com, kvm@vger.kernel.org,
        iommu@lists.linux-foundation.org, dwmw2@infradead.org,
        linux-kernel@vger.kernel.org, baolu.lu@linux.intel.com,
        david@gibson.dropbear.id.au, nicolinc@nvidia.com
Subject: Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE
Message-ID: <YUszZRk1vZOgVvFF@myrica>
References: <20210919063848.1476776-1-yi.l.liu@intel.com>
 <20210919063848.1476776-12-yi.l.liu@intel.com>
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On Sun, Sep 19, 2021 at 02:38:39PM +0800, Liu Yi L wrote:
> This patch adds IOASID allocation/free interface per iommufd. When
> allocating an IOASID, userspace is expected to specify the type and
> format information for the target I/O page table.
> 
> This RFC supports only one type (IOMMU_IOASID_TYPE_KERNEL_TYPE1V2),
> implying a kernel-managed I/O page table with vfio type1v2 mapping
> semantics. For this type the user should specify the addr_width of
> the I/O address space and whether the I/O page table is created in
> an iommu enfore_snoop format. enforce_snoop must be true at this point,
> as the false setting requires additional contract with KVM on handling
> WBINVD emulation, which can be added later.
> 
> Userspace is expected to call IOMMU_CHECK_EXTENSION (see next patch)
> for what formats can be specified when allocating an IOASID.
> 
> Open:
> - Devices on PPC platform currently use a different iommu driver in vfio.
>   Per previous discussion they can also use vfio type1v2 as long as there
>   is a way to claim a specific iova range from a system-wide address space.

Is this the reason for passing addr_width to IOASID_ALLOC?  I didn't get
what it's used for or why it's mandatory. But for PPC it sounds like it
should be an address range instead of an upper limit?

Thanks,
Jean

>   This requirement doesn't sound PPC specific, as addr_width for pci devices
>   can be also represented by a range [0, 2^addr_width-1]. This RFC hasn't
>   adopted this design yet. We hope to have formal alignment in v1 discussion
>   and then decide how to incorporate it in v2.
> 
> - Currently ioasid term has already been used in the kernel (drivers/iommu/
>   ioasid.c) to represent the hardware I/O address space ID in the wire. It
>   covers both PCI PASID (Process Address Space ID) and ARM SSID (Sub-Stream
>   ID). We need find a way to resolve the naming conflict between the hardware
>   ID and software handle. One option is to rename the existing ioasid to be
>   pasid or ssid, given their full names still sound generic. Appreciate more
>   thoughts on this open!