Re: [PATCH v6 12/19] vfio-user: IOMMU support for remote device

[Stefan Hajnoczi]

On Thu, Mar 03, 2022 at 02:49:53PM +0000, Jag Raman wrote:
> 
> 
> > On Mar 2, 2022, at 11:49 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Mon, Feb 28, 2022 at 07:54:38PM +0000, Jag Raman wrote:
> >> 
> >> 
> >>> On Feb 22, 2022, at 5:40 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>> 
> >>> On Thu, Feb 17, 2022 at 02:48:59AM -0500, Jagannathan Raman wrote:
> >>>> +struct RemoteIommuElem {
> >>>> +    AddressSpace  as;
> >>>> +    MemoryRegion  mr;
> >>>> +};
> >>>> +
> >>>> +GHashTable *remote_iommu_elem_by_bdf;
> >>> 
> >>> A mutable global hash table requires synchronization when device
> >>> emulation runs in multiple threads.
> >>> 
> >>> I suggest using pci_setup_iommu()'s iommu_opaque argument to avoid the
> >>> global. If there is only 1 device per remote PCI bus, then there are no
> >>> further synchronization concerns.
> >> 
> >> OK, will avoid the global. We would need to access the hash table
> >> concurrently since there could be more than one device in the
> >> same bus - so a mutex would be needed here.
> > 
> > I thought the PCIe topology can be set up with a separate buf for each
> > x-vfio-user-server? I remember something like that in the previous
> > revision where a root port was instantiated for each x-vfio-user-server.
> 
> Yes, we could setup the PCIe topology to be that way. But the user could
> add more than one device to the same bus, unless the bus type explicitly
> limits the number of devices to one (BusClass->max_dev).

Due to how the IOMMU is used to restrict the bus to the vfio-user
client's DMA mappings, it seems like it's necesssary to limit the number
of devices to 1 per bus anyway?

Stefan

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