Message-ID: <20180320165514.0f4cd556@t450s.home>
Date:   Tue, 20 Mar 2018 16:55:14 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "pmorel@...ux.vnet.ibm.com" <pmorel@...ux.vnet.ibm.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xuwei5@...ilicon.com" <xuwei5@...ilicon.com>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v5 0/7] vfio/type1: Add support for valid iova list
 management

On Mon, 19 Mar 2018 08:28:32 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Shameer Kolothum
> > Sent: Friday, March 16, 2018 12:35 AM
> > 
> > This series introduces an iova list associated with a vfio
> > iommu. The list is kept updated taking care of iommu apertures,
> > and reserved regions. Also this series adds checks for any conflict
> > with existing dma mappings whenever a new device group is attached to
> > the domain.
> > 
> > User-space can retrieve valid iova ranges using VFIO_IOMMU_GET_INFO
> > ioctl capability chains. Any dma map request outside the valid iova
> > range will be rejected.  
> 
> GET_INFO is done at initialization time which is good for cold attached
> devices. If a hotplugged device may cause change of valid iova ranges
> at run-time, then there could be potential problem (which however is
> difficult for user space or orchestration stack to figure out in advance)
> Can we do some extension like below to make hotplug case cleaner?

Let's be clear what we mean by hotplug here, as I see it, the only
relevant hotplug would be a physical device, hot added to the host,
which becomes a member of an existing, in-use IOMMU group.  If, on the
other hand, we're talking about hotplug related to the user process,
there's nothing asynchronous about that.  For instance in the QEMU
case, QEMU must add the group to the container, at which point it can
evaluate the new iova list and remove the group from the container if
it doesn't like the result.  So what would be a case of the available
iova list for a group changing as a result of adding a device?

> - An interface allowing user space to request VFIO rejecting further 
> attach_group if doing so may cause iova range change. e.g. Qemu can 
> do such request once completing initial GET_INFO;

For the latter case above, it seems unnecessary, QEMU can revert the
attach, we're only promising that the attach won't interfere with
existing mappings.  For the host hotplug case, the user has no control,
the new device is a member of the iommu group and therefore necessarily
becomes a part of container.  I imagine there are plenty of other holes
in this scenario already.
 
> - or an event notification to user space upon change of valid iova
> ranges when attaching a new device at run-time. It goes one step 
> further - even attach may cause iova range change, it may still
> succeed as long as Qemu hasn't allocated any iova in impacted 
> range

Same as above, in the QEMU hotplug case, the user is orchestrating the
adding of the group to the container, they can then check the iommu
info on their own and determine what, if any, changes are relevant to
them, knowing that the addition would not succeed if any current
mappings where affected.  In the host case, a notification would be
required, but we'd first need to identify exactly how the iova list can
change asynchronous to the user doing anything.  Thanks,

Alex

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