Re: [PATCH v1 15/22] Add iommufd configure option

["Cédric Le Goater" <clg@redhat.com>]

On 9/20/23 14:51, Jason Gunthorpe wrote:
> On Wed, Sep 20, 2023 at 02:19:42PM +0200, Cédric Le Goater wrote:
>> On 9/20/23 05:42, Duan, Zhenzhong wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Cédric Le Goater <clg@redhat.com>
>>>> Sent: Wednesday, September 20, 2023 1:08 AM
>>>> Subject: Re: [PATCH v1 15/22] Add iommufd configure option
>>>>
>>>> On 8/30/23 12:37, Zhenzhong Duan wrote:
>>>>> This adds "--enable-iommufd/--disable-iommufd" to enable or disable
>>>>> iommufd support, enabled by default.
>>>>
>>>> Why would someone want to disable support at compile time ? It might
>>>
>>> For those users who only want to support legacy container feature?
>>> Let me know if you still prefer to drop this patch, I'm fine with that.
>>
>> I think it is too early.
>>
>>>> have been useful for dev but now QEMU should self-adjust at runtime
>>>> depending only on the host capabilities AFAIUI. Am I missing something ?
>>>
>>> IOMMUFD doesn't support all features of legacy container, so QEMU
>>> doesn't self-adjust at runtime by checking if host supports IOMMUFD.
>>> We need to specify it explicitly to use IOMMUFD as below:
>>>
>>>       -object iommufd,id=iommufd0
>>>       -device vfio-pci,host=0000:02:00.0,iommufd=iommufd0
>>
>> OK. I am not sure this is the correct interface yet. At first glance,
>> I wouldn't introduce a new object for a simple backend depending on a
>> kernel interface. I would tend to prefer a "iommu-something" property
>> of the vfio-pci device with string values: "legacy", "iommufd", "default"
>> and define the various interfaces (the ops you proposed) for each
>> depending on the user preference and the capabilities of the host and
>> possibly the device.
> 
> I think the idea came from Alex? The major point is to be able to have
> libvirt open /dev/iommufd and FD pass it into qemu 

ok.

> and then share that single FD across all VFIOs. 

I will ask Alex to help me catch up on the topic.

> qemu will typically not be able to
> self-open /dev/iommufd as it is root-only.

I don't understand, we open multiple fds to KVM devices. This is the same.

> 
> So the object is not exactly for the backend, the object is for the
> file descriptor.
got it.

> 
> Adding a legacy/iommufd option to the vfio-pci device string doesn't
> address these needs.

I agree.

Thanks,

C.

> Jason
>