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Subject: Re: [RFC PATCH v1 0/4] vfio: Add IOPF support for VFIO passthrough
To:     Keqian Zhu <zhukeqian1@huawei.com>,
        Shenming Lu <lushenming@huawei.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>
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From:   Lu Baolu <baolu.lu@linux.intel.com>
Message-ID: <993b34b0-a68e-87eb-854d-ce926db8702b@linux.intel.com>
Date:   Sat, 20 Mar 2021 09:35:55 +0800
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On 3/19/21 9:30 AM, Keqian Zhu wrote:
> 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.

Make sense. Thanks a lot for the explanation.

> 
> 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.

Best regards,
baolu