netdev - Re: [RFC 3/7] vhost: Add 3 new uapi to support iommufd

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Message-ID: <20230925134959.GD13733@nvidia.com>
Date: Mon, 25 Sep 2023 10:49:59 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Cindy Lu <lulu@...hat.com>
Cc: jasowang@...hat.com, mst@...hat.com, yi.l.liu@...el.com,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [RFC 3/7] vhost: Add 3 new uapi to support iommufd

On Sun, Sep 24, 2023 at 01:05:36AM +0800, Cindy Lu wrote:

> +/* vhost vdpa set iommufd
> + * Input parameters:
> + * @iommufd: file descriptor from /dev/iommu; pass -1 to unset
> + * @group_id: identifier of the group that a virtqueue belongs to
> + * @ioas_id: IOAS identifier returned from ioctl(IOMMU_IOAS_ALLOC)
> + * Output parameters:
> + * @out_dev_id: device identifier
> + * @out_hwpt_id: hardware IO pagetable identifier
> + */
> +struct vhost_vdpa_set_iommufd {
> +	__s32 iommufd;
> +	__u32 group_id;
> +	__u32 ioas_id;
> +	__u32 out_dev_id;
> +	__u32 out_hwpt_id;
> +};
> +
> +#define VHOST_VDPA_SET_IOMMU_FD \
> +	_IOW(VHOST_VIRTIO, 0x7e, struct vhost_vdpa_set_iommufd)
> +
> +/*
> + * VDPA_DEVICE_ATTACH_IOMMUFD_AS -
> + * _IOW(VHOST_VIRTIO, 0x7f, struct vdpa_device_attach_iommufd_as)
> + *
> + * Attach a vdpa device to an iommufd address space specified by IOAS
> + * id.
> + *
> + * Available only after a device has been bound to iommufd via
> + * VHOST_VDPA_SET_IOMMU_FD
> + *
> + * Undo by VDPA_DEVICE_DETACH_IOMMUFD_AS or device fd close.
> + *
> + * @argsz:	user filled size of this data.
> + * @flags:	must be 0.
> + * @ioas_id:	Input the target id which can represent an ioas
> + *		allocated via iommufd subsystem.
> + *
> + * Return: 0 on success, -errno on failure.
> + */
> +struct vdpa_device_attach_iommufd_as {
> +	__u32 argsz;
> +	__u32 flags;
> +	__u32 ioas_id;
> +};

I don't know this at all, but it seems really weird that the API
taking in the iommfd is more specific (eg includes the group_id) than
the API taking in the ioas.

I'd expect that you have one iommfd setup per virtio FD.

And the various virtio queues would be able to be linked to their own
IOAS.

I'm not sure multiple iommufd's per virito FD is worthwhile.

Jason