Re: [PATCH v5 03/18] pci: isolated address space for PCI bus

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jag Raman (jag.raman@oracle.com) wrote:
> 
> 
> > On Jan 25, 2022, at 1:38 PM, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> > 
> > * Jag Raman (jag.raman@oracle.com) wrote:
> >> 
> >> 
> >>> On Jan 19, 2022, at 7:12 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >>> 
> >>> On Wed, Jan 19, 2022 at 04:41:52PM -0500, Jagannathan Raman wrote:
> >>>> Allow PCI buses to be part of isolated CPU address spaces. This has a
> >>>> niche usage.
> >>>> 
> >>>> TYPE_REMOTE_MACHINE allows multiple VMs to house their PCI devices in
> >>>> the same machine/server. This would cause address space collision as
> >>>> well as be a security vulnerability. Having separate address spaces for
> >>>> each PCI bus would solve this problem.
> >>> 
> >>> Fascinating, but I am not sure I understand. any examples?
> >> 
> >> Hi Michael!
> >> 
> >> multiprocess QEMU and vfio-user implement a client-server model to allow
> >> out-of-process emulation of devices. The client QEMU, which makes ioctls
> >> to the kernel and runs VCPUs, could attach devices running in a server
> >> QEMU. The server QEMU needs access to parts of the client’s RAM to
> >> perform DMA.
> > 
> > Do you ever have the opposite problem? i.e. when an emulated PCI device
> 
> That’s an interesting question.
> 
> > exposes a chunk of RAM-like space (frame buffer, or maybe a mapped file)
> > that the client can see.  What happens if two emulated devices need to
> > access each others emulated address space?
> 
> In this case, the kernel driver would map the destination’s chunk of internal RAM into
> the DMA space of the source device. Then the source device could write to that
> mapped address range, and the IOMMU should direct those writes to the
> destination device.

Are all devices mappable like that?

> I would like to take a closer look at the IOMMU implementation on how to achieve
> this, and get back to you. I think the IOMMU would handle this. Could you please
> point me to the IOMMU implementation you have in mind?

I didn't have one in mind; I was just hitting a similar problem on
Virtiofs DAX.

Dave

> Thank you!
> --
> Jag
> 
> > 
> > Dave
> > 
> >> In the case where multiple clients attach devices that are running on the
> >> same server, we need to ensure that each devices has isolated memory
> >> ranges. This ensures that the memory space of one device is not visible
> >> to other devices in the same server.
> >> 
> >>> 
> >>> I also wonder whether this special type could be modelled like a special
> >>> kind of iommu internally.
> >> 
> >> Could you please provide some more details on the design?
> >> 
> >>> 
> >>>> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> >>>> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> >>>> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> >>>> ---
> >>>> include/hw/pci/pci.h     |  2 ++
> >>>> include/hw/pci/pci_bus.h | 17 +++++++++++++++++
> >>>> hw/pci/pci.c             | 17 +++++++++++++++++
> >>>> hw/pci/pci_bridge.c      |  5 +++++
> >>>> 4 files changed, 41 insertions(+)
> >>>> 
> >>>> diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> >>>> index 023abc0f79..9bb4472abc 100644
> >>>> --- a/include/hw/pci/pci.h
> >>>> +++ b/include/hw/pci/pci.h
> >>>> @@ -387,6 +387,8 @@ void pci_device_save(PCIDevice *s, QEMUFile *f);
> >>>> int pci_device_load(PCIDevice *s, QEMUFile *f);
> >>>> MemoryRegion *pci_address_space(PCIDevice *dev);
> >>>> MemoryRegion *pci_address_space_io(PCIDevice *dev);
> >>>> +AddressSpace *pci_isol_as_mem(PCIDevice *dev);
> >>>> +AddressSpace *pci_isol_as_io(PCIDevice *dev);
> >>>> 
> >>>> /*
> >>>> * Should not normally be used by devices. For use by sPAPR target
> >>>> diff --git a/include/hw/pci/pci_bus.h b/include/hw/pci/pci_bus.h
> >>>> index 347440d42c..d78258e79e 100644
> >>>> --- a/include/hw/pci/pci_bus.h
> >>>> +++ b/include/hw/pci/pci_bus.h
> >>>> @@ -39,9 +39,26 @@ struct PCIBus {
> >>>>    void *irq_opaque;
> >>>>    PCIDevice *devices[PCI_SLOT_MAX * PCI_FUNC_MAX];
> >>>>    PCIDevice *parent_dev;
> >>>> +
> >>>>    MemoryRegion *address_space_mem;
> >>>>    MemoryRegion *address_space_io;
> >>>> 
> >>>> +    /**
> >>>> +     * Isolated address spaces - these allow the PCI bus to be part
> >>>> +     * of an isolated address space as opposed to the global
> >>>> +     * address_space_memory & address_space_io.
> >>> 
> >>> Are you sure address_space_memory & address_space_io are
> >>> always global? even in the case of an iommu?
> >> 
> >> On the CPU side of the Root Complex, I believe address_space_memory
> >> & address_space_io are global.
> >> 
> >> In the vfio-user case, devices on the same machine (TYPE_REMOTE_MACHINE)
> >> could be attached to different clients VMs. Each client would have their own address
> >> space for their CPUs. With isolated address spaces, we ensure that the devices
> >> see the address space of the CPUs they’re attached to.
> >> 
> >> Not sure if it’s OK to share weblinks in this mailing list, please let me know if that’s
> >> not preferred. But I’m referring to the terminology used in the following block diagram:
> >> https://en.wikipedia.org/wiki/Root_complex#/media/File:Example_PCI_Express_Topology.svg
> >> 
> >>> 
> >>>> This allows the
> >>>> +     * bus to be attached to CPUs from different machines. The
> >>>> +     * following is not used used commonly.
> >>>> +     *
> >>>> +     * TYPE_REMOTE_MACHINE allows emulating devices from multiple
> >>>> +     * VM clients,
> >>> 
> >>> what are VM clients?
> >> 
> >> It’s the client in the client - server model explained above.
> >> 
> >> Thank you!
> >> --
> >> Jag
> >> 
> >>> 
> >>>> as such it needs the PCI buses in the same machine
> >>>> +     * to be part of different CPU address spaces. The following is
> >>>> +     * useful in that scenario.
> >>>> +     *
> >>>> +     */
> >>>> +    AddressSpace *isol_as_mem;
> >>>> +    AddressSpace *isol_as_io;
> >>>> +
> >>>>    QLIST_HEAD(, PCIBus) child; /* this will be replaced by qdev later */
> >>>>    QLIST_ENTRY(PCIBus) sibling;/* this will be replaced by qdev later */
> >>>> 
> >>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> >>>> index 5d30f9ca60..d5f1c6c421 100644
> >>>> --- a/hw/pci/pci.c
> >>>> +++ b/hw/pci/pci.c
> >>>> @@ -442,6 +442,8 @@ static void pci_root_bus_internal_init(PCIBus *bus, DeviceState *parent,
> >>>>    bus->slot_reserved_mask = 0x0;
> >>>>    bus->address_space_mem = address_space_mem;
> >>>>    bus->address_space_io = address_space_io;
> >>>> +    bus->isol_as_mem = NULL;
> >>>> +    bus->isol_as_io = NULL;
> >>>>    bus->flags |= PCI_BUS_IS_ROOT;
> >>>> 
> >>>>    /* host bridge */
> >>>> @@ -2676,6 +2678,16 @@ MemoryRegion *pci_address_space_io(PCIDevice *dev)
> >>>>    return pci_get_bus(dev)->address_space_io;
> >>>> }
> >>>> 
> >>>> +AddressSpace *pci_isol_as_mem(PCIDevice *dev)
> >>>> +{
> >>>> +    return pci_get_bus(dev)->isol_as_mem;
> >>>> +}
> >>>> +
> >>>> +AddressSpace *pci_isol_as_io(PCIDevice *dev)
> >>>> +{
> >>>> +    return pci_get_bus(dev)->isol_as_io;
> >>>> +}
> >>>> +
> >>>> static void pci_device_class_init(ObjectClass *klass, void *data)
> >>>> {
> >>>>    DeviceClass *k = DEVICE_CLASS(klass);
> >>>> @@ -2699,6 +2711,7 @@ static void pci_device_class_base_init(ObjectClass *klass, void *data)
> >>>> 
> >>>> AddressSpace *pci_device_iommu_address_space(PCIDevice *dev)
> >>>> {
> >>>> +    AddressSpace *iommu_as = NULL;
> >>>>    PCIBus *bus = pci_get_bus(dev);
> >>>>    PCIBus *iommu_bus = bus;
> >>>>    uint8_t devfn = dev->devfn;
> >>>> @@ -2745,6 +2758,10 @@ AddressSpace *pci_device_iommu_address_space(PCIDevice *dev)
> >>>>    if (!pci_bus_bypass_iommu(bus) && iommu_bus && iommu_bus->iommu_fn) {
> >>>>        return iommu_bus->iommu_fn(bus, iommu_bus->iommu_opaque, devfn);
> >>>>    }
> >>>> +    iommu_as = pci_isol_as_mem(dev);
> >>>> +    if (iommu_as) {
> >>>> +        return iommu_as;
> >>>> +    }
> >>>>    return &address_space_memory;
> >>>> }
> >>>> 
> >>>> diff --git a/hw/pci/pci_bridge.c b/hw/pci/pci_bridge.c
> >>>> index da34c8ebcd..98366768d2 100644
> >>>> --- a/hw/pci/pci_bridge.c
> >>>> +++ b/hw/pci/pci_bridge.c
> >>>> @@ -383,6 +383,11 @@ void pci_bridge_initfn(PCIDevice *dev, const char *typename)
> >>>>    sec_bus->address_space_io = &br->address_space_io;
> >>>>    memory_region_init(&br->address_space_io, OBJECT(br), "pci_bridge_io",
> >>>>                       4 * GiB);
> >>>> +
> >>>> +    /* This PCI bridge puts the sec_bus in its parent's address space */
> >>>> +    sec_bus->isol_as_mem = pci_isol_as_mem(dev);
> >>>> +    sec_bus->isol_as_io = pci_isol_as_io(dev);
> >>>> +
> >>>>    br->windows = pci_bridge_region_init(br);
> >>>>    QLIST_INIT(&sec_bus->child);
> >>>>    QLIST_INSERT_HEAD(&parent->child, sec_bus, sibling);
> >>>> -- 
> >>>> 2.20.1
> >> 
> > -- 
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK