Re: [PATCH] docs: Add '-device intel-iommu' entry

[Eric Auger <eauger@redhat.com>]

Hi Peter,

On 6/11/21 8:55 PM, Peter Xu wrote:
> The parameters of intel-iommu device are non-trivial to understand.  Add an
> entry for it so that people can reference to it when using.
> 
> There're actually a few more options there, but I hide them explicitly because
> they shouldn't be used by normal QEMU users.
> 
> Cc: Chao Yang <chayang@redhat.com>
> Cc: Lei Yang <leiyang@redhat.com>
> Cc: Jing Zhao <jinzhao@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  qemu-options.hx | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 14258784b3a..4bb04243907 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -926,6 +926,38 @@ SRST
>  
>  ``-device pci-ipmi-bt,bmc=id``
>      Like the KCS interface, but defines a BT interface on the PCI bus.
> +
> +``-device intel-iommu[,option=...]``
> +    This is only supported by ``-machine q35``, which will enable Intel VT-d
> +    emulation within the guest.  It supports below options:
> +
> +    ``intremap=on|off`` (default: auto)
> +        This enables interrupt remapping feature in the guest.  It's required
> +        to enable complete x2apic.  Currently it only supports kvm
> +        kernel-irqchip modes ``off`` or ``split``.  Full kernel-irqchip is not
> +        yet supported.
maybe explain how the default is chosen (based on kernel-irqhcip mode?)
> +
> +    ``caching-mode=on|off`` (default: off)
> +        This enables caching mode for the VT-d emulated device.  When
> +        caching-mode is enabled, each guest DMA buffer mapping will generate an
> +        IOTLB invalidation from the guest IOMMU driver to the vIOMMU device in
> +        a synchronous way.  It is required for ``-device vfio-pci`` to work
> +        with the VT-d device, because host assigned devices requires to setup
> +        the DMA mapping on the host before guest DMA starts.
> +
> +    ``device-iotlb=on|off`` (default: off)
> +        This enables device-iotlb capability for the emulated VT-d device.  So
> +        far virtio/vhost should be the only real user for this parameter,
I thought if was needed for vhost. What is the usage for virtio?
> +        paired with ats=on configured for the device.
What is not obvious to me is whether there is a kind of diagnosis
whether device-iotlb and caching mode settings are consistent with
vfio/vhost usage. Does qemu exit if there is inconsistency?
> +
> +    ``aw-bits=39|48`` (default: 39)
> +        This decides the address width of IOVA address space.  The address
> +        space has 39 bits width for 3-level IOMMU page tables, and 48 bits for
> +        4-level IOMMU page tables.
> +
> +    Please also refer to the wiki page for general scenarios of VT-d
> +    emulation in QEMU: https://wiki.qemu.org/Features/VT-d.
> +
>  ERST
>  
>  DEF("name", HAS_ARG, QEMU_OPTION_name,
>
Besides, it is quite useful.

Thanks

Eric