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

[Stefan Hajnoczi]

On Wed, Mar 30, 2022 at 01:13:03PM -0400, Peter Xu wrote:
> On Wed, Mar 30, 2022 at 05:08:24PM +0100, Stefan Hajnoczi wrote:
> > 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.
> 
> It can't, and we only support one vIOMMU so far for both (commit
> 1b3bf13890fd849b26).  Thanks,

I see, thanks!

Okay, summarizing options for the vfio-user IOMMU:

1. Use the same singleton approach as existing IOMMUs where the
   MemoryRegion/AddressSpace are never freed. Don't bother deleting.

2. Keep the approach in this patch where vfio-user code manually calls a
   custom delete function (not part of the pci_setup_iommu() API). This
   is slightly awkward to do without global state and that's what
   started this discussion.

3. Introduce an optional pci_setup_iommu() callback:

   typdef void (*PCIIOMMUDeviceUnplug)(PCIBus *bus, void *opaque, int devfn);

   Solves the awkwardness of option #2. Not needed by existing IOMMU
   devices.

Any preferences anyone?

Stefan

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