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

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Message-ID: <a535a91a-3af7-b43d-8399-01255a070f2b@linux.intel.com>
Date:   Fri, 19 Mar 2021 08:33:27 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Shenming Lu <lushenming@...wei.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Cornelia Huck <cohuck@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Eric Auger <eric.auger@...hat.com>,
        "wanghaibin.wang@...wei.com" <wanghaibin.wang@...wei.com>,
        "yuzenghui@...wei.com" <yuzenghui@...wei.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>
Subject: Re: [RFC PATCH v1 0/4] vfio: Add IOPF support for VFIO passthrough

On 3/18/21 7:53 PM, Shenming Lu wrote:
> On 2021/3/18 17:07, Tian, Kevin wrote:
>>> From: Shenming Lu <lushenming@...wei.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.

> 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