Re: [RFC PATCH v1 0/4] vfio: Add IOPF support for VFIO passthrough

[Shenming Lu <lushenming@huawei.com>]

On 2021/3/18 17:07, Tian, Kevin wrote:
>> From: Shenming Lu <lushenming@huawei.com>
>> Sent: Thursday, March 18, 2021 3:53 PM
>>
>> On 2021/2/4 14:52, Tian, Kevin wrote:>>> In reality, many
>>>>> devices allow I/O faulting only in selective contexts. However, there
>>>>> is no standard way (e.g. PCISIG) for the device to report whether
>>>>> arbitrary I/O fault is allowed. Then we may have to maintain device
>>>>> specific knowledge in software, e.g. in an opt-in table to list devices
>>>>> which allows arbitrary faults. For devices which only support selective
>>>>> faulting, a mediator (either through vendor extensions on vfio-pci-core
>>>>> or a mdev wrapper) might be necessary to help lock down non-faultable
>>>>> mappings and then enable faulting on the rest mappings.
>>>>
>>>> For devices which only support selective faulting, they could tell it to the
>>>> IOMMU driver and let it filter out non-faultable faults? Do I get it wrong?
>>>
>>> Not exactly to IOMMU driver. There is already a vfio_pin_pages() for
>>> selectively page-pinning. The matter is that 'they' imply some device
>>> specific logic to decide which pages must be pinned and such knowledge
>>> is outside of VFIO.
>>>
>>> From enabling p.o.v we could possibly do it in phased approach. First
>>> handles devices which tolerate arbitrary DMA faults, and then extends
>>> to devices with selective-faulting. The former is simpler, but with one
>>> main open whether we want to maintain such device IDs in a static
>>> table in VFIO or rely on some hints from other components (e.g. PF
>>> driver in VF assignment case). Let's see how Alex thinks about it.
>>
>> Hi Kevin,
>>
>> You mentioned selective-faulting some time ago. I still have some doubt
>> about it:
>> There is already a vfio_pin_pages() which is used for limiting the IOMMU
>> group dirty scope to pinned pages, could it also be used for indicating
>> the faultable scope is limited to the pinned pages and the rest mappings
>> is non-faultable that should be pinned and mapped immediately? But it
>> seems to be a little weird and not exactly to what you meant... I will
>> be grateful if you can help to explain further. :-)
>>
> 
> The opposite, i.e. the vendor driver uses vfio_pin_pages to lock down
> pages that are not faultable (based on its specific knowledge) and then
> the rest memory becomes faultable.

Ahh...
Thus, from the perspective of VFIO IOMMU, if IOPF enabled for such device,
only the page faults within the pinned range are valid in the registered
iommu fault handler...
I have another question here, for the IOMMU backed devices, they are already
all pinned and mapped when attaching, is there a need to call vfio_pin_pages()
to lock down pages for them? Did I miss something?...

Thanks,
Shenming

> 
> Thanks
> Kevin
>