linux-kernel - Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices

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Date:   Thu, 27 Oct 2016 08:30:10 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>, <pbonzini@...hat.com>,
        <kraxel@...hat.com>, <cjia@...dia.com>, <jike.song@...el.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kevin.tian@...el.com>, <qemu-devel@...gnu.org>,
        <bjsdjshi@...ux.vnet.ibm.com>
Subject: Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for
 mediated devices

On Thu, 27 Oct 2016 18:01:51 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 10/27/2016 12:50 PM, Alexey Kardashevskiy wrote:
> > On 18/10/16 08:22, Kirti Wankhede wrote:  
> >> VFIO IOMMU drivers are designed for the devices which are IOMMU capable.
> >> Mediated device only uses IOMMU APIs, the underlying hardware can be
> >> managed by an IOMMU domain.
> >>
> >> Aim of this change is:
> >> - To use most of the code of TYPE1 IOMMU driver for mediated devices
> >> - To support direct assigned device and mediated device in single module
> >>
> >> Added two new callback functions to struct vfio_iommu_driver_ops. Backend
> >> IOMMU module that supports pining and unpinning pages for mdev devices
> >> should provide these functions.
> >> Added APIs for pining and unpining pages to VFIO module. These calls back
> >> into backend iommu module to actually pin and unpin pages.
> >>
> >> This change adds pin and unpin support for mediated device to TYPE1 IOMMU
> >> backend module. More details:
> >> - When iommu_group of mediated devices is attached, task structure is
> >>   cached which is used later to pin pages and page accounting.  
> > 
> > 
> > For SPAPR TCE IOMMU driver, I ended up caching mm_struct with
> > atomic_inc(&container->mm->mm_count) (patches are on the way) instead of
> > using @current or task as the process might be gone while VFIO container is
> > still alive and @mm might be needed to do proper cleanup; this might not be
> > an issue with this patchset now but still you seem to only use @mm from
> > task_struct.
> >   
> 
> Consider the example of QEMU process which creates VFIO container, QEMU
> in its teardown path would release the container. How could container be
> alive when process is gone?

If QEMU is sent a SIGKILL, does the process still exist?  We must be
able to perform cleanup regardless of the state, or existence, of the
task that created it.  Thanks,

Alex