linux-kernel - Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu

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Date:   Thu, 12 Nov 2020 10:31:55 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Zeng, Xin" <xin.zeng@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu_ops for IOMMU
 callbacks

Hi Alex,

On 11/3/20 1:22 PM, Lu Baolu wrote:
> Hi Alex,
> 
> On 10/31/20 5:06 AM, Alex Williamson wrote:
>> On Fri, 30 Oct 2020 06:16:28 +0000
>> "Tian, Kevin" <kevin.tian@...el.com> wrote:
>>
>>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>>> Sent: Friday, October 30, 2020 12:58 PM
>>>>
>>>> With the IOMMU driver registering iommu_ops for the mdev_bus, the
>>>> IOMMU
>>>> operations on an mdev could be done in the same way as any normal 
>>>> device
>>>> (for example, PCI/PCIe). There's no need to distinguish an mdev from
>>>> others for iommu operations. Remove the unnecessary code.
>>>
>>> This is really a nice cleanup as the output of this change! :)
>>
>> It's easy to remove a bunch of code when the result is breaking
>> everyone else.  Please share with me how SR-IOV backed mdevs continue
>> to work on AMD platforms, or how they might work on ARM platforms, when
>> siov_iommu_ops (VT-d only) becomes the one and only provider of
>> iommu_ops on the mdev bus.  Hard NAK on this series.  Thanks,
> 
> I focused too much on a feature and forgot about university. I should
> apologize for this. Sorry about it!
> 
> Back to the original intention of this series. The aux domain was
> allocated in vfio/mdev, but it's also needed by the vDCM component of a
> device driver for mediated callbacks. Currently vfio/mdev or iommu core
> has no support for this.
> 
> We had a proposal when we first did aux-domain support. But was not
> discussed since there was no consumer at that time.
> 
> https://lore.kernel.org/linux-iommu/20181105073408.21815-7-baolu.lu@linux.intel.com/ 

Exposing iommu_domain outside of the vfio/iommu abstract seems not a
secure idea. I have posted a new proposal. Can you please help to
review?

https://lore.kernel.org/linux-iommu/20201112022407.2063896-1-baolu.lu@linux.intel.com/

Best regards.
baolu

> 
> 
> Does it look good to you? I can send patches of such solution for
> discussion if you think it's a right way.
> 
> Extending the iommu core for subdevice passthrough support sounds an
> interesting topic, but it will take much time before we reach a
> consensus. It sounds a good topic for the next year's LPC/MC :-).
> 
> Best regards,
> baolu