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Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
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To:     Alex Williamson <alex.williamson@redhat.com>
CC:     Jason Gunthorpe <jgg@nvidia.com>,
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From:   Kirti Wankhede <kwankhede@nvidia.com>
Message-ID: <6f9dad1a-4e15-1703-d6f4-1b51b7fde15a@nvidia.com>
Date:   Wed, 26 May 2021 23:40:02 +0530
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On 5/26/2021 4:22 AM, Alex Williamson wrote:
> On Wed, 26 May 2021 00:56:30 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 5/25/2021 5:07 AM, Jason Gunthorpe wrote:
>>> On Mon, May 24, 2021 at 05:52:58PM +1000, David Gibson wrote:
>>>    
>>>>>> I don't really see a semantic distinction between "always one-device
>>>>>> groups" and "groups don't matter".  Really the only way you can afford
>>>>>> to not care about groups is if they're singletons.
>>>>>
>>>>> The kernel driver under the mdev may not be in an "always one-device"
>>>>> group.
>>>>
>>>> I don't really understand what you mean by that.
>>>
>>> I mean the group of the mdev's actual DMA device may have multiple
>>> things in it.
>>>      
>>>>> It is a kernel driver so the only thing we know and care about is that
>>>>> all devices in the HW group are bound to kernel drivers.
>>>>>
>>>>> The vfio device that spawns from this kernel driver is really a
>>>>> "groups don't matter" vfio device because at the IOMMU layer it should
>>>>> be riding on the physical group of the kernel driver.  At the VFIO
>>>>> layer we no longer care about the group abstraction because the system
>>>>> guarentees isolation in some other way.
>>>>
>>>> Uh.. I don't really know how mdevs are isolated from each other.  I
>>>> thought it was because the physical device providing the mdevs
>>>> effectively had an internal IOMMU (or at least DMA permissioning) to
>>>> isolate the mdevs, even though the physical device may not be fully
>>>> isolated.
>>>>
>>>> In that case the virtual mdev is effectively in a singleton group,
>>>> which is different from the group of its parent device.
>>>    
>>
>> That's correct.
>>
>>> That is one way to view it, but it means creating a whole group
>>> infrastructure and abusing the IOMMU stack just to create this
>>> nonsense fiction.
>>
>> I really didn't get how this abuse the IOMMU stack.
>> mdev can be used in 3 different ways:
>> 1. non-iommu backed mdev devices where mdev vendor driver takes care to
>> DMA map (iommu_map) and isolation is through device hardware internal
>> MMU. Here vfio_iommu_type1 module provides a way to validate and pin
>> pages required by mdev device for DMA mapping. Then IOMMU mapping is
>> done by mdev vendor driver which is owner driver of physical device.
>>
>> 2. iommu backed mdev devices for SRIOV where mdev device is created per
>> VF (mdev device == VF device) then that mdev device has same iommu
>> protection scope as VF associated to it. Here mdev device is virtual
>> device which uses features of mdev and represents underlying VF device,
>> same as vfio-pci but with additional mdev features.
> 
> What features would those be?  There are no mdev specific parts of the
> vfio uAPI.
> 
> The mdev device is a virtual device, by why it it virtual in this case?
> Aren't we effectively assigning the VF itself (mdev device == VF device)
> with a bunch of extra support code to fill in the gaps of the VF
> implementing the complete device model in hardware?
> 
> We're effectively creating this virtual device, creating a fake IOMMU
> group, and trying to create this association of this virtual device to
> the real VF in order to shoehorn it into the mdev model.  What do we
> get from that model other than lifecycle management (ie. type selection)
> and re-use of a bunch of code from the driver supporting the 1) model
> above?
> 

Yes, the lifecycle management which is in mdev is not in vfio-pci variant.

> This specific model seems better served by a device specific peer
> driver to vfio-pci (ie. a "vfio-pci variant").  You effectively already
> have the code for this driver, it's just in the format of an mdev
> driver rather than a vfio "bus driver".  The work Jason references
> relative to Max aims to make these kinds of drivers easier to implement
> through re-use of vfio-pci code.
> 
> There are certainly other solutions we could come up with for selecting
> a specific device type for a vfio-pci variant driver to implement other
> than pretending this model actually belongs in mdev, right?  Thanks,
> 

Sure and would like to see type selection mechanism to be implemented in 
vfio-pci variant.

Thanks,
Kirti