Re: [Qemu-devel] [RFC 1/5] hw/vfio: Add function for getting reserved_region of device iommu group

[Auger Eric]

Hi Shameer,

On 06/12/17 11:30, Shameerali Kolothum Thodi wrote:
> Hi Alex,
> 
>> -----Original Message-----
>> From: Shameerali Kolothum Thodi
>> Sent: Monday, November 20, 2017 4:31 PM
>> To: 'Alex Williamson' <address@hidden>
>> Cc: address@hidden; Zhuyijun <address@hidden>; qemu-
>> address@hidden; address@hidden; address@hidden;
>> Zhaoshenglong <address@hidden>; Linuxarm
>> <address@hidden>
>> Subject: RE: [Qemu-devel] [RFC 1/5] hw/vfio: Add function for getting
>> reserved_region of device iommu group
> [...]
>>>>> And sysfs is a good interface if the user wants to use it to
>>>>> configure the VM in a way that's compatible with a device.  For
>>>>> instance, in your case, a user could evaluate these reserved
>>>>> regions across all devices in a system, or even across an entire
>>>>> cluster, and instantiate the VM with a memory map compatible with
>>>>> hotplugging any of those evaluated devices (QEMU implementation of
>> allowing the user to do this TBD).
>>>>> Having the vfio device evaluate these reserved regions only helps
>>>>> in the cold-plug case.  So the proposed solution is limited in
>>>>> scope and doesn't address similar needs on other platforms.  There
>>>>> is value to verifying that a device's IOVA space is compatible
>>>>> with a VM memory map and modifying the memory map on cold-plug or
>>>>> rejecting the device on hot-plug, but isn't that why we have an
>>>>> ioctl within vfio to expose information about the IOMMU?  Why take
>>>>> the path of allowing QEMU to rummage through sysfs files outside
>>>>> of vfio, implying additional security and access concerns, rather
>>>>> than filling the gap within the vifo API?
>>>>
>>>> Thanks Alex for the explanation.
>>>>
>>>> I came across this patch[1] from Eric where he introduced the IOCTL
>>> interface to
>>>> retrieve the reserved regions. It looks like this can be reworked to
>>> accommodate
>>>> the above requirement.
>>>
>>> I don't think we need a new ioctl for this, nor do I think that
>>> describing the holes is the correct approach.  The existing
>>> VFIO_IOMMU_GET_INFO ioctl can be extended to support capability
>>> chains, as we've done for VFIO_DEVICE_GET_REGION_INFO.
>>
>> Right, as far as I can see the above mentioned patch is doing exactly the 
>> same,
>> extending the VFIO_IOMMU_GET_INFO ioctl with capability chain.
>>
>>> IMO, we should try to
>>> describe the available fixed IOVA regions which are available for
>>> mapping rather than describing various holes within the address space
>>> which are unavailable.  The latter method always fails to describe the
>>> end of the mappable IOVA space and gets bogged down in trying to
>>> classify the types of holes that might exist.  Thanks,
>>
> 
> I was going through this and noticed that it is possible to have multiple 
> iommu domains associated with a container. If that's true, is it safe to
> make the assumption that all the domains will have the same iova
> geometry or not?
To me the answer is no.

There are several iommu domains "underneath" a single container. You
attach vfio groups to a container. Each of them is associated to an
iommu group and an iommu domain. See vfio_iommu_type1_attach_group().

Besides, the reserved regions are per iommu group.

Thanks

Eric

> 
> Thanks,
> Shameer
>