From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: kvm@vger.kernel.org, rafael@kernel.org,
David Airlie <airlied@linux.ie>,
linux-pci@vger.kernel.org,
Thierry Reding <thierry.reding@gmail.com>,
Diana Craciun <diana.craciun@oss.nxp.com>,
Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
Ashok Raj <ashok.raj@intel.com>,
Jonathan Hunter <jonathanh@nvidia.com>,
Christoph Hellwig <hch@infradead.org>,
Stuart Yoder <stuyoder@gmail.com>,
Kevin Tian <kevin.tian@intel.com>,
Chaitanya Kulkarni <kch@nvidia.com>,
Alex Williamson <alex.williamson@redhat.com>,
Bjorn Helgaas <bhelgaas@google.com>,
Dan Williams <dan.j.williams@intel.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Cornelia Huck <cohuck@redhat.com>,
linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
iommu@lists.linux-foundation.org,
Jacob jun Pan <jacob.jun.pan@intel.com>,
Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()
Date: Mon, 14 Feb 2022 16:38:23 +0000 [thread overview]
Message-ID: <f302e823-ecc3-2aae-e275-85a56e26fb25@arm.com> (raw)
In-Reply-To: <20220214145627.GD4160@nvidia.com>
On 2022-02-14 14:56, Jason Gunthorpe via iommu wrote:
> On Mon, Feb 14, 2022 at 02:10:19PM +0000, Robin Murphy wrote:
>> On 2022-02-14 12:45, Jason Gunthorpe wrote:
>>> On Mon, Feb 14, 2022 at 12:09:36PM +0000, Robin Murphy wrote:
>>>> On 2022-01-06 02:20, Lu Baolu wrote:
>>>>> Expose an interface to replace the domain of an iommu group for frameworks
>>>>> like vfio which claims the ownership of the whole iommu group.
>>>>
>>>> But if the underlying point is the new expectation that
>>>> iommu_{attach,detach}_device() operate on the device's whole group where
>>>> relevant, why should we invent some special mechanism for VFIO to be
>>>> needlessly inconsistent?
>>>>
>>>> I said before that it's trivial for VFIO to resolve a suitable device if it
>>>> needs to; by now I've actually written the patch ;)
>>>>
>>>> https://gitlab.arm.com/linux-arm/linux-rm/-/commit/9f37d8c17c9b606abc96e1f1001c0b97c8b93ed5
>>>
>>> Er, how does locking work there? What keeps busdev from being
>>> concurrently unplugged?
>>
>> Same thing that prevents the bus pointer from suddenly becoming invalid in
>> the current code, I guess :)
>
> Oooh, yes, that does look broken now too. :(
>
>>> How can iommu_group_get() be safely called on
>>> this pointer?
>>
>> What matters is being able to call *other* device-based IOMMU API
>> interfaces in the long term.
>
> Yes, this is what I mean, those are the ones that call
> iommu_group_get().
>
>>> All of the above only works normally inside a probe/remove context
>>> where the driver core is blocking concurrent unplug and descruction.
>>>
>>> I think I said this last time you brought it up that lifetime was the
>>> challenge with this idea.
>>
>> Indeed, but it's a challenge that needs tackling, because the bus-based
>> interfaces need to go away. So either we figure it out now and let this
>> attach interface rework benefit immediately, or I spend three times as long
>
> IMHO your path is easier if you let VFIO stay with the group interface
> and use something like:
>
> domain = iommu_group_alloc_domain(group)
>
> Which is what VFIO is trying to accomplish. Since Lu removed the only
> other user of iommu_group_for_each_dev() it means we can de-export
> that interface.
>
> This works better because the iommu code can hold the internal group
> while it finds the bus/device and then invokes the driver op. We don't
> have a lifetime problem anymore under that lock.
That's certainly one of the cleaner possibilities - per the theme of
this thread I'm not hugely keen on proliferating special VFIO-specific
versions of IOMMU APIs, but trying to take the dev->mutex might be a bit
heavy-handed and risky, and getting at the vfio_group->device_lock a bit
fiddly, so if I can't come up with anything nicer or more general it
might be a fair compromise.
> The remaining VFIO use of bus for iommu_capable() is better done
> against the domain or the group object, as appropriate.
Indeed, although half the implementations of .capable are nonsense
already, so I'm treating that one as a secondary priority for the moment
(with an aim to come back afterwards and just try to kill it off as far
as possible). RDMA and VFIO shouldn't be a serious concern for the kind
of systems with heterogeneous IOMMUs at this point.
> In the bigger picture, VFIO should stop doing
> 'iommu_group_alloc_domain' by moving the domain alloc to
> VFIO_GROUP_GET_DEVICE_FD where we have a struct device to use.
>
> We've already been experimenting with this for iommufd and the subtle
> difference in the uapi doesn't seem relevant.
>
>> solving it on my own and end up deleting
>> iommu_group_replace_domain() in about 6 months' time anyway.
>
> I expect this API to remain until we figure out a solution to the PPC
> problem, and come up with an alternative way to change the attached
> domain on the fly.
I though PPC wasn't using the IOMMU API at all... or is that the problem?
Thanks,
Robin.
next prev parent reply other threads:[~2022-02-14 16:41 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-06 2:20 [PATCH v1 0/8] Scrap iommu_attach/detach_group() interfaces Lu Baolu
2022-01-06 2:20 ` [PATCH v1 1/8] iommu: Add iommu_group_replace_domain() Lu Baolu
2022-01-06 17:06 ` Jason Gunthorpe
2022-01-07 0:26 ` Lu Baolu
2022-02-14 12:09 ` Robin Murphy
2022-02-14 12:45 ` Jason Gunthorpe
2022-02-14 14:10 ` Robin Murphy
2022-02-14 14:56 ` Jason Gunthorpe
2022-02-14 16:38 ` Robin Murphy [this message]
2022-02-14 17:25 ` Jason Gunthorpe
2022-01-06 2:20 ` [PATCH v1 2/8] vfio/type1: Use iommu_group_replace_domain() Lu Baolu
2022-01-06 2:20 ` [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for multi-device groups Lu Baolu
2022-01-06 17:22 ` Jason Gunthorpe
2022-01-07 1:14 ` Lu Baolu
2022-01-07 1:19 ` Jason Gunthorpe
2022-02-14 11:39 ` Joerg Roedel
2022-02-14 13:03 ` Jason Gunthorpe
2022-02-14 14:39 ` Joerg Roedel
2022-02-14 15:18 ` Robin Murphy
2022-02-14 15:46 ` Jason Gunthorpe
2022-02-15 8:58 ` Joerg Roedel
2022-02-15 13:47 ` Jason Gunthorpe
2022-02-16 6:28 ` Lu Baolu
2022-02-16 13:54 ` Jason Gunthorpe
2022-01-06 2:20 ` [PATCH v1 4/8] drm/tegra: Use iommu_attach/detatch_device() Lu Baolu
2022-01-06 2:20 ` [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device() Lu Baolu
2022-01-06 14:33 ` Jason Gunthorpe
2022-01-07 0:23 ` Lu Baolu
2022-02-14 11:27 ` Joerg Roedel
2022-02-14 13:15 ` Jason Gunthorpe
2022-02-14 13:40 ` Joerg Roedel
2022-02-14 14:02 ` Jason Gunthorpe
2022-02-14 14:23 ` Joerg Roedel
2022-02-14 15:00 ` Jason Gunthorpe
2022-02-15 9:11 ` Joerg Roedel
2022-02-15 13:02 ` Robin Murphy
2022-02-15 14:37 ` Jason Gunthorpe
2022-01-06 2:20 ` [PATCH v1 6/8] gpu/host1x: " Lu Baolu
2022-01-06 15:35 ` Jason Gunthorpe
2022-01-07 0:35 ` Lu Baolu
2022-01-07 0:48 ` Jason Gunthorpe
2022-01-07 1:19 ` Lu Baolu
2022-01-06 2:20 ` [PATCH v1 7/8] media: staging: media: tegra-vde: " Lu Baolu
2022-01-06 2:20 ` [PATCH v1 8/8] iommu: Remove iommu_attach/detach_group() Lu Baolu
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