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

[Eric Auger <eauger@redhat.com>]

Hi Peter,
On 7/7/21 5:35 PM, Peter Xu wrote:
> On Wed, Jul 07, 2021 at 11:29:47AM +0200, Eric Auger wrote:
>> Hi Peter,
> 
> Hi, Eric,
> 
>>
>> 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?)
> 
> Will do.
> 
>>> +
>>> +    ``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?
> 
> I used "virtio/vhost" because vhost is only one type of virtio backend, so
> normally they should be used in pair.  Also, the "ats=on" parameter should be
> an option to virtio frontend rather than vhost, hence I mentioned both.

OK makes sense.
> 
>>> +        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?
> 
> Caching mode check is done in vtd_machine_done_notify_one().
> 
> I think there's no such check for vhost, however iirc vhost should work
> with/without ats=on; I did't check deeper.
OK
> 
>>> +
>>> +    ``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 for taking a look!
> 
Besides,

Reviewed-by: Eric Auger <eric.auger@redhat.com>


Eric