Re: [PATCH v7 12/17] vfio-user: IOMMU support for remote device

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Mar 30, 2022 at 08:53:16AM -0400, Peter Xu wrote:
> On Wed, Mar 30, 2022 at 11:04:24AM +0100, Stefan Hajnoczi wrote:
> > This makes me wonder whether there is a deeper issue with the
> > pci_setup_iommu() API: the lack of per-device cleanup callbacks.
> > Per-device IOMMU resources should be freed when a device is hot
> > unplugged.
> > 
> > From what I can tell this is not the case today:
> > 
> > - hw/i386/intel_iommu.c:vtd_find_add_as() allocates and adds device
> >   address spaces but I can't find where they are removed and freed.
> >   VTDAddressSpace instances pointed to from vtd_bus->dev_as[] are leaked.
> > 
> > - hw/i386/amd_iommu.c has similar leaks.
> 
> AFAICT it's because there's no device-specific data cached in the
> per-device IOMMU address space, at least so far.  IOW, all the data
> structures allocated here can be re-used when a new device is plugged in
> after the old device unplugged.
> 
> It's definitely not ideal since after unplug (and before a new device
> plugged in) the resource is not needed at all so it's kind of wasted, but
> it should work functionally.  If to achieve that, some iommu_unplug() or
> iommu_cleanup() hook sounds reasonable.

I guess the question is whether PCI busses can be hotplugged with
IOMMUs. If yes, then there is a memory leak that matters for
intel_iommu.c and amd_iommu.c.

Stefan

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