linux-kernel - Re: [PATCH v5 02/15] iommu: Report domain nesting info

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Message-ID: <99c20ada-b7e8-44f6-e036-ab905d119119@redhat.com>
Date:   Fri, 17 Jul 2020 18:29:28 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Liu Yi L <yi.l.liu@...el.com>, alex.williamson@...hat.com,
        baolu.lu@...ux.intel.com, joro@...tes.org
Cc:     kevin.tian@...el.com, jacob.jun.pan@...ux.intel.com,
        ashok.raj@...el.com, jun.j.tian@...el.com, yi.y.sun@...el.com,
        jean-philippe@...aro.org, peterx@...hat.com, hao.wu@...el.com,
        stefanha@...il.com, iommu@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 02/15] iommu: Report domain nesting info

Hi Yi,

On 7/12/20 1:20 PM, Liu Yi L wrote:
> IOMMUs that support nesting translation needs report the capability info
s/needs/need to report
> to userspace, e.g. the format of first level/stage paging structures.
It gives information about requirements the userspace needs to implement
plus other features characterizing the physical implementation.
> 
> This patch reports nesting info by DOMAIN_ATTR_NESTING. Caller can get
> nesting info after setting DOMAIN_ATTR_NESTING.
I guess you meant after selecting VFIO_TYPE1_NESTING_IOMMU?
> 
> Cc: Kevin Tian <kevin.tian@...el.com>
> CC: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Cc: Alex Williamson <alex.williamson@...hat.com>
> Cc: Eric Auger <eric.auger@...hat.com>
> Cc: Jean-Philippe Brucker <jean-philippe@...aro.org>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Lu Baolu <baolu.lu@...ux.intel.com>
> Signed-off-by: Liu Yi L <yi.l.liu@...el.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
> v4 -> v5:
> *) address comments from Eric Auger.
> 
> v3 -> v4:
> *) split the SMMU driver changes to be a separate patch
> *) move the @addr_width and @pasid_bits from vendor specific
>    part to generic part.
> *) tweak the description for the @features field of struct
>    iommu_nesting_info.
> *) add description on the @data[] field of struct iommu_nesting_info
> 
> v2 -> v3:
> *) remvoe cap/ecap_mask in iommu_nesting_info.
> *) reuse DOMAIN_ATTR_NESTING to get nesting info.
> *) return an empty iommu_nesting_info for SMMU drivers per Jean'
>    suggestion.
> ---
>  include/uapi/linux/iommu.h | 77 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 77 insertions(+)
> 
> diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
> index 1afc661..d2a47c4 100644
> --- a/include/uapi/linux/iommu.h
> +++ b/include/uapi/linux/iommu.h
> @@ -332,4 +332,81 @@ struct iommu_gpasid_bind_data {
>  	} vendor;
>  };
>  
> +/*
> + * struct iommu_nesting_info - Information for nesting-capable IOMMU.
> + *			       user space should check it before using
> + *			       nesting capability.
> + *
> + * @size:	size of the whole structure
> + * @format:	PASID table entry format, the same definition as struct
> + *		iommu_gpasid_bind_data @format.
> + * @features:	supported nesting features.
> + * @flags:	currently reserved for future extension.
> + * @addr_width:	The output addr width of first level/stage translation
> + * @pasid_bits:	Maximum supported PASID bits, 0 represents no PASID
> + *		support.
> + * @data:	vendor specific cap info. data[] structure type can be deduced
> + *		from @format field.
> + *
> + * +===============+======================================================+
> + * | feature       |  Notes                                               |
> + * +===============+======================================================+
> + * | SYSWIDE_PASID |  PASIDs are managed in system-wide, instead of per   |
s/in system-wide/system-wide ?
> + * |               |  device. When a device is assigned to userspace or   |
> + * |               |  VM, proper uAPI (userspace driver framework uAPI,   |
> + * |               |  e.g. VFIO) must be used to allocate/free PASIDs for |
> + * |               |  the assigned device.          
Isn't it possible to be more explicit, something like:
                      |
System-wide PASID management is mandated by the physical IOMMU. All
PASIDs allocation must be mediated through the TBD API.
> + * +---------------+------------------------------------------------------+
> + * | BIND_PGTBL    |  The owner of the first level/stage page table must  |
> + * |               |  explicitly bind the page table to associated PASID  |
> + * |               |  (either the one specified in bind request or the    |
> + * |               |  default PASID of iommu domain), through userspace   |
> + * |               |  driver framework uAPI (e.g. VFIO_IOMMU_NESTING_OP). |
As per your answer in https://lkml.org/lkml/2020/7/6/383, I now
understand ARM would not expose that BIND_PGTBL nesting feature, I still
think the above wording is a bit confusing. Maybe you may explicitly
talk about the PASID *entry* that needs to be passed from guest to host.
On ARM we directly pass the PASID table but when reading the above
description I fail to determine if this does not fit that description.
> + * +---------------+------------------------------------------------------+
> + * | CACHE_INVLD   |  The owner of the first level/stage page table must  |
> + * |               |  explicitly invalidate the IOMMU cache through uAPI  |
> + * |               |  provided by userspace driver framework (e.g. VFIO)  |
> + * |               |  according to vendor-specific requirement when       |
> + * |               |  changing the page table.                            |
> + * +---------------+------------------------------------------------------+

instead of using the "uAPI provided by userspace driver framework (e.g.
VFIO)", can't we use the so-called IOMMU UAPI terminology which now has
a userspace documentation?

> + *
> + * @data[] types defined for @format:
> + * +================================+=====================================+
> + * | @format                        | @data[]                             |
> + * +================================+=====================================+
> + * | IOMMU_PASID_FORMAT_INTEL_VTD   | struct iommu_nesting_info_vtd       |
> + * +--------------------------------+-------------------------------------+
> + *
> + */
> +struct iommu_nesting_info {
> +	__u32	size;
shouldn't it be @argsz to fit the iommu uapi convention and take benefit
to put the flags field just below?
> +	__u32	format;
> +#define IOMMU_NESTING_FEAT_SYSWIDE_PASID	(1 << 0)
> +#define IOMMU_NESTING_FEAT_BIND_PGTBL		(1 << 1)
> +#define IOMMU_NESTING_FEAT_CACHE_INVLD		(1 << 2)
> +	__u32	features;
> +	__u32	flags;
> +	__u16	addr_width;
> +	__u16	pasid_bits;
> +	__u32	padding;
> +	__u8	data[];
> +};
> +
> +/*
> + * struct iommu_nesting_info_vtd - Intel VT-d specific nesting info
> + *
> + * @flags:	VT-d specific flags. Currently reserved for future
> + *		extension.
must be set to 0?
> + * @cap_reg:	Describe basic capabilities as defined in VT-d capability
> + *		register.
> + * @ecap_reg:	Describe the extended capabilities as defined in VT-d
> + *		extended capability register.
> + */
> +struct iommu_nesting_info_vtd {
> +	__u32	flags;
> +	__u32	padding;
> +	__u64	cap_reg;
> +	__u64	ecap_reg;
> +};
> +
>  #endif /* _UAPI_IOMMU_H */
Thanks

Eric
>