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

[Keqian Zhu <zhukeqian1@huawei.com>]

Hi Baolu,

On 2021/3/19 8:33, Lu Baolu wrote:
> On 3/18/21 7:53 PM, Shenming Lu wrote:
>> 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...
> 
> Isn't it opposite? The pinned pages will never generate any page faults.
> I might miss some contexts here.
It seems that vfio_pin_pages() just pin some pages and record the pinned scope to pfn_list of vfio_dma.
No mapping is established, so we still has page faults.

IIUC, vfio_pin_pages() is used to
1. pin pages for non-iommu backed devices.
2. mark dirty scope for non-iommu backed devices and iommu backed devices.

Thanks,
Keqian

> 
>> 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?...
> 
> Best regards,
> baolu
> .
>