linux-kernel - Re: [PATCH v4 04/22] iommu/vt-d: add bind_pasid

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Date:   Thu, 31 May 2018 10:09:46 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Yi L <yi.l.liu@...ux.intel.com>,
        Auger Eric <eric.auger@...hat.com>
Subject: Re: [PATCH v4 04/22] iommu/vt-d: add bind_pasid_table function

On 30/05/18 20:52, Jacob Pan wrote:
>> However I think the model number should be added to
>> pasid_table_config. For one thing it gives us a simple sanity-check,
>> but it also tells which other fields are valid in pasid_table_config.
>> Arm-smmu-v3 needs at least two additional 8-bit fields describing the
>> PASID table format (number of levels and PASID0 behaviour), which are
>> written to device context tables when installing the PASID table
>> pointer.
>>
> We had model number field in v2 of this patchset. My thought was that
> since the config info is meant to be generic, we shouldn't include
> model info. But I also think a simple sanity check can be useful,
> would that be sufficient to address Alex's concern? Of course we still
> need sysfs for more specific IOMMU features.
> 
> Would this work?
> enum pasid_table_model {
> 	PASID_TABLE_FORMAT_HOST,
> 	PASID_TABLE_FORMAT_ARM_1LVL,
> 	PASID_TABLE_FORMAT_ARM_2LVL,

I'd rather use a single PASID_TABLE_FORMAT_ARM, because "2LVL" may be
further split into 2LVL_4k or 2LVL_64k leaf tables... I think it's best
if I add an arch-specific field in pasid_table_config for that, and for
the PASID0 configuration, when adding FORMAT_ARM in a future patch

> 	PASID_TABLE_FORMAT_AMD,
> 	PASID_TABLE_FORMAT_INTEL,
> };
> 
> /**
>  * PASID table data used to bind guest PASID table to the host IOMMU. This will
>  * enable guest managed first level page tables.
>  * @version: for future extensions and identification of the data format
>  * @bytes: size of this structure
>  * @model: PASID table format for different IOMMU models
>  * @base_ptr:	PASID table pointer
>  * @pasid_bits:	number of bits supported in the guest PASID table, must be less
>  *		or equal than the host supported PASID size.
>  */
> struct pasid_table_config {
> 	__u32 version;
> #define PASID_TABLE_CFG_VERSION_1 1
> 	__u32 bytes;

"bytes" could be passed by VFIO as argument to bind_pasid_table, since
it can deduce it from argsz

Thanks,
Jean

> 	enum pasid_table_model model;
> 	__u64 base_ptr;
> 	__u8 pasid_bits;
> };
> 
> 
> 
>> Compatibility: new optional features are easy to add to a given model,
>> just add a new sysfs file. If in the future, the host describes a new
>> feature that is mandatory, or implements a different PASID table
>> format, how does it ensure that user understands it? Perhaps use a
>> new model number for this, e.g. "arm-smmu-v3-a=3", with similar
>> features. I think it would be the same if the host stops supporting a
>> feature for a given model, because they are ABI. But we can also
>> define default values from the start, for example "if ssid_bits file
>> isn't present, default value is 0 - PASID not supported"
>>
>> Thanks,
>> Jean
> 
> [Jacob Pan]
>