Re: [PATCH v5 03/15] iommu/smmu: Report empty domain nesting info

[Auger Eric <eric.auger@redhat.com>]

Hi Jean,

On 7/17/20 11:09 AM, Jean-Philippe Brucker wrote:
> On Thu, Jul 16, 2020 at 10:38:17PM +0200, Auger Eric wrote:
>> Hi Jean,
>>
>> On 7/16/20 5:39 PM, Jean-Philippe Brucker wrote:
>>> On Tue, Jul 14, 2020 at 10:12:49AM +0000, Liu, Yi L wrote:
>>>>> Have you verified that this doesn't break the existing usage of
>>>>> DOMAIN_ATTR_NESTING in drivers/vfio/vfio_iommu_type1.c?
>>>>
>>>> I didn't have ARM machine on my hand. But I contacted with Jean
>>>> Philippe, he confirmed no compiling issue. I didn't see any code
>>>> getting DOMAIN_ATTR_NESTING attr in current drivers/vfio/vfio_iommu_type1.c.
>>>> What I'm adding is to call iommu_domai_get_attr(, DOMAIN_ATTR_NESTIN)
>>>> and won't fail if the iommu_domai_get_attr() returns 0. This patch
>>>> returns an empty nesting info for DOMAIN_ATTR_NESTIN and return
>>>> value is 0 if no error. So I guess it won't fail nesting for ARM.
>>>
>>> I confirm that this series doesn't break the current support for
>>> VFIO_IOMMU_TYPE1_NESTING with an SMMUv3. That said...
>>>
>>> If the SMMU does not support stage-2 then there is a change in behavior
>>> (untested): after the domain is silently switched to stage-1 by the SMMU
>>> driver, VFIO will now query nesting info and obtain -ENODEV. Instead of
>>> succeding as before, the VFIO ioctl will now fail. I believe that's a fix
>>> rather than a regression, it should have been like this since the
>>> beginning. No known userspace has been using VFIO_IOMMU_TYPE1_NESTING so
>>> far, so I don't think it should be a concern.
>> But as Yi mentioned ealier, in the current vfio code there is no
>> DOMAIN_ATTR_NESTING query yet.
> 
> That's why something that would have succeeded before will now fail:
> Before this series, if user asked for a VFIO_IOMMU_TYPE1_NESTING, it would
> have succeeded even if the SMMU didn't support stage-2, as the driver
> would have silently fallen back on stage-1 mappings (which work exactly
> the same as stage-2-only since there was no nesting supported). After the
> series, we do check for DOMAIN_ATTR_NESTING so if user asks for
> VFIO_IOMMU_TYPE1_NESTING and the SMMU doesn't support stage-2, the ioctl
> fails.
OK I now understand what you meant. Yes this actual change is brought by
the next patch, ie.
[PATCH v5 04/15] vfio/type1: Report iommu nesting info to userspace

Thanks

Eric
> 
> I believe it's a good fix and completely harmless, but wanted to make sure
> no one objects because it's an ABI change.
> 
> Thanks,
> Jean
> 
>> In my SMMUV3 nested stage series, I added
>> such a query in vfio-pci.c to detect if I need to expose a fault region
>> but I already test both the returned value and the output arg. So to me
>> there is no issue with that change.
>>>
>>> And if userspace queries the nesting properties using the new ABI
>>> introduced in this patchset, it will obtain an empty struct. I think
>>> that's acceptable, but it may be better to avoid adding the nesting cap if
>>> @format is 0?
>> agreed
>>
>> Thanks
>>
>> Eric
>>>
>>> Thanks,
>>> Jean
>>>
>>>>
>>>> @Eric, how about your opinion? your dual-stage vSMMU support may
>>>> also share the vfio_iommu_type1.c code.
>>>>
>>>> Regards,
>>>> Yi Liu
>>>>
>>>>> Will
>>>
>>
>