Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu_ops for IOMMU callbacks

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Alex Williamson <alex.williamson@redhat.com>,
	"Tian, Kevin" <kevin.tian@intel.com>
Cc: baolu.lu@linux.intel.com, Joerg Roedel <joro@8bytes.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Cornelia Huck <cohuck@redhat.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	"Liu, Yi L" <yi.l.liu@intel.com>,
	"Zeng, Xin" <xin.zeng@intel.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu_ops for IOMMU callbacks
Date: Tue, 3 Nov 2020 13:22:05 +0800	[thread overview]
Message-ID: <5802fc4a-7dc0-eda7-4e7c-809bcec6bd90@linux.intel.com> (raw)
In-Reply-To: <20201030150625.2dc5fb9b@w520.home>

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@intel.com> wrote:
> 
>>> From: Lu Baolu <baolu.lu@linux.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/

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