linux-kernel - Re: [PATCH v2 14/15] vfio: Document dual stage control

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Message-ID: <20200615094128.GB1491454@stefanha-x1.localdomain>
Date:   Mon, 15 Jun 2020 10:41:28 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Liu Yi L <yi.l.liu@...el.com>
Cc:     alex.williamson@...hat.com, eric.auger@...hat.com,
        baolu.lu@...ux.intel.com, joro@...tes.org, 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,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 14/15] vfio: Document dual stage control

On Thu, Jun 11, 2020 at 05:15:33AM -0700, Liu Yi L wrote:
> From: Eric Auger <eric.auger@...hat.com>
> 
> The VFIO API was enhanced to support nested stage control: a bunch of
> new iotcls and usage guideline.
> 
> Let's document the process to follow to set up nested mode.
> 
> 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: Eric Auger <eric.auger@...hat.com>
> Signed-off-by: Liu Yi L <yi.l.liu@...el.com>
> ---
> v1 -> v2:
> *) new in v2, compared with Eric's original version, pasid table bind
>    and fault reporting is removed as this series doesn't cover them.
>    Original version from Eric.
>    https://lkml.org/lkml/2020/3/20/700
> 
>  Documentation/driver-api/vfio.rst | 64 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/Documentation/driver-api/vfio.rst b/Documentation/driver-api/vfio.rst
> index f1a4d3c..06224bd 100644
> --- a/Documentation/driver-api/vfio.rst
> +++ b/Documentation/driver-api/vfio.rst
> @@ -239,6 +239,70 @@ group and can access them as follows::
>  	/* Gratuitous device reset and go... */
>  	ioctl(device, VFIO_DEVICE_RESET);
>  
> +IOMMU Dual Stage Control
> +------------------------
> +
> +Some IOMMUs support 2 stages/levels of translation. Stage corresponds to
> +the ARM terminology while level corresponds to Intel's VTD terminology.
> +In the following text we use either without distinction.
> +
> +This is useful when the guest is exposed with a virtual IOMMU and some
> +devices are assigned to the guest through VFIO. Then the guest OS can use
> +stage 1 (GIOVA -> GPA or GVA->GPA), while the hypervisor uses stage 2 for
> +VM isolation (GPA -> HPA).
> +
> +Under dual stage translation, the guest gets ownership of the stage 1 page
> +tables and also owns stage 1 configuration structures. The hypervisor owns
> +the root configuration structure (for security reason), including stage 2
> +configuration. This works as long configuration structures and page table

s/as long configuration/as long as configuration/

> +format are compatible between the virtual IOMMU and the physical IOMMU.

s/format/formats/

> +
> +Assuming the HW supports it, this nested mode is selected by choosing the
> +VFIO_TYPE1_NESTING_IOMMU type through:
> +
> +    ioctl(container, VFIO_SET_IOMMU, VFIO_TYPE1_NESTING_IOMMU);
> +
> +This forces the hypervisor to use the stage 2, leaving stage 1 available
> +for guest usage. The guest stage 1 format depends on IOMMU vendor, and
> +it is the same with the nesting configuration method. User space should
> +check the format and configuration method after setting nesting type by
> +using:
> +
> +    ioctl(container->fd, VFIO_IOMMU_GET_INFO, &nesting_info);
> +
> +Details can be found in Documentation/userspace-api/iommu.rst. For Intel
> +VT-d, each stage 1 page table is bound to host by:
> +
> +    nesting_op->flags = VFIO_IOMMU_NESTING_OP_BIND_PGTBL;
> +    memcpy(&nesting_op->data, &bind_data, sizeof(bind_data));
> +    ioctl(container->fd, VFIO_IOMMU_NESTING_OP, nesting_op);
> +
> +As mentioned above, guest OS may use stage 1 for GIOVA->GPA or GVA->GPA.
> +GVA->GPA page tables are available when PASID (Process Address Space ID)
> +is exposed to guest. e.g. guest with PASID-capable devices assigned. For
> +such page table binding, the bind_data should include PASID info, which
> +is allocated by guest itself or by host. This depends on hardware vendor
> +e.g. Intel VT-d requires to allocate PASID from host. This requirement is
> +available by VFIO_IOMMU_GET_INFO. User space could allocate PASID from
> +host by:
> +
> +    req.flags = VFIO_IOMMU_ALLOC_PASID;
> +    ioctl(container, VFIO_IOMMU_PASID_REQUEST, &req);

It is not clear how the userspace application determines whether PASIDs
must be allocated from the host via VFIO_IOMMU_PASID_REQUEST or if the
guest itself can allocate PASIDs. The text mentions VFIO_IOMMU_GET_INFO
but what exactly should the userspace application check?

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