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Re: [Qemu-ppc] Proposal PCI/PCIe device placement on PAPR guests

From: Greg Kurz
Subject: Re: [Qemu-ppc] Proposal PCI/PCIe device placement on PAPR guests
Date: Thu, 12 Jan 2017 10:09:03 +0100

On Thu, 12 Jan 2017 17:19:40 +1100
Alexey Kardashevskiy <address@hidden> wrote:

> On 12/01/17 14:52, David Gibson wrote:
> > On Fri, Jan 06, 2017 at 12:57:58PM +0100, Greg Kurz wrote:  
> >> On Thu, 5 Jan 2017 16:46:18 +1100
> >> David Gibson <address@hidden> wrote:
> >>  
> >>> There was a discussion back in November on the qemu list which spilled
> >>> onto the libvirt list about how to add support for PCIe devices to
> >>> POWER VMs, specifically 'pseries' machine type PAPR guests.
> >>>
> >>> Here's a more concrete proposal for how to handle part of this in
> >>> future from the libvirt side.  Strictly speaking what I'm suggesting
> >>> here isn't intrinsically linked to PCIe: it will make adding PCIe
> >>> support sanely easier, as well as having a number of advantages for
> >>> both PCIe and plain-PCI devices on PAPR guests.
> >>>
> >>> Background:
> >>>
> >>>  * Currently the pseries machine type only supports vanilla PCI
> >>>    buses.
> >>>     * This is a qemu limitation, not something inherent - PAPR guests
> >>>       running under PowerVM (the IBM hypervisor) can use passthrough
> >>>       PCIe devices (PowerVM doesn't emulate devices though).
> >>>     * In fact the way PCI access is para-virtalized in PAPR makes the
> >>>       usual distinctions between PCI and PCIe largely disappear
> >>>  * Presentation of PCIe devices to PAPR guests is unusual
> >>>     * Unlike x86 - and other "bare metal" platforms, root ports are
> >>>       not made visible to the guest. i.e. all devices (typically)
> >>>       appear as though they were integrated devices on x86
> >>>     * In terms of topology all devices will appear in a way similar to
> >>>       a vanilla PCI bus, even PCIe devices
> >>>        * However PCIe extended config space is accessible
> >>>     * This means libvirt's usual placement of PCIe devices is not
> >>>       suitable for PAPR guests
> >>>  * PAPR has its own hotplug mechanism
> >>>     * This is used instead of standard PCIe hotplug
> >>>     * This mechanism works for both PCIe and vanilla-PCI devices
> >>>     * This can hotplug/unplug devices even without a root port P2P
> >>>       bridge between it and the root "bus
> >>>  * Multiple independent host bridges are routine on PAPR
> >>>     * Unlike PC (where all host bridges have multiplexed access to
> >>>       configuration space) PCI host bridges (PHBs) are truly
> >>>       independent for PAPR guests (disjoint MMIO regions in system
> >>>       address space)
> >>>     * PowerVM typically presents a separate PHB to the guest for each
> >>>       host slot passed through
> >>>
> >>> The Proposal:
> >>>
> >>> I suggest that libvirt implement a new default algorithm for placing
> >>> (i.e. assigning addresses to) both PCI and PCIe devices for (only)
> >>> PAPR guests.
> >>>
> >>> The short summary is that by default it should assign each device to a
> >>> separate vPHB, creating vPHBs as necessary.
> >>>
> >>>   * For passthrough sometimes a group of host devices can't be safely
> >>>     isolated from each other - this is known as a (host) Partitionable
> >>>     Endpoint (PE).  In this case, if any device in the PE is passed
> >>>     through to a guest, the whole PE must be passed through to the
> >>>     same vPHB in the guest.  From the guest POV, each vPHB has exactly
> >>>     one (guest) PE.
> >>>   * To allow for hotplugged devices, libvirt should also add a number
> >>>     of additional, empty vPHBs (the PAPR spec allows for hotplug of
> >>>     PHBs, but this is not yet implemented in qemu).  When hotplugging
> >>>     a new device (or PE) libvirt should locate a vPHB which doesn't
> >>>     currently contain anything.
> >>>   * libvirt should only (automatically) add PHBs - never root ports or
> >>>     other PCI to PCI bridges
> >>>
> >>> In order to handle migration, the vPHBs will need to be represented in
> >>> the domain XML, which will also allow the user to override this
> >>> topology if they want.
> >>>
> >>> Advantages:
> >>>
> >>> There are still some details I need to figure out w.r.t. handling PCIe
> >>> devices (on both the qemu and libvirt sides).  However the fact that  
> >>
> >> One such detail may be that PCIe devices should have the
> >> "ibm,pci-config-space-type" property set to 1 in the DT,
> >> for the driver to be able to access the extended config
> >> space.  
> > 
> > So, we have a bit of an oddity here.  It looks like we currently set
> > 'ibm,pci-config-space-type' to 1 in the PHB, rather than individual
> > device nodes.  Which, AFAICT, is simply incorrect in terms of PAPR.  
> I asked Paul how to read the spec and this is rather correct but not enough
> - having type=1 on a PHB means that extended access requests can go behind
> it but underlying devices and bridges still need to have type=1 if they
> support extended space. Having type set to 0 (or none at all) on a PHB
> would mean that extended config space is not available on anything under
> this PHB.

I have the very same understanding of the spec (LoPAPR March 2016):

R1–9.1.8–2. All IOAs that implement PCI-X Mode 2 or PCI Express must supply the 
fig-space-type” property (see Section B.‚ “Properties for Children of 
PCI Host Bridges‚” on
page 703).

Implementation Note: The “ibm,pci-config-space-type” property in Requirement 
R1–9.1.8–2 is added for
platforms that support I/O fabric and IOAs that implement PCI-X Mode 2, and PCI 
Express. To access the
extended configuration space provided by PCI-X Mode 2 and PCI Express, all I/O 
fabric leading up to an IOA
must support a 12-bit register number. In other words, if a platform 
implementation has a conventional PCI bridge
leading up to an IOA that implements PCI-X Mode 2, the platform will not be 
able to provide access to the
extended configuration space of that IOA. The “ibm,config-space-type” property 
in the IOA's OF node
is used by device drivers to determine if an IOA’s extended configuration space 
can be accessed.


B. Properties for Children of PCI Host Bridges

property name: Indicates if the platform supports access to an extended 
configuration address space from the PHB
up to and including this node.
0 = Platform supports only an eight bit register number for configuration 
address space accesses.
1 = Platform supports a twelve bit register number for configuration address 
space accesses.
This property may be provided in all PHB nodes and their children.
Note: The absence of this property implies the platform supports only an eight 
bit register number for configura-
tion address space accesses.

And incidentally, this is what the linux kernel currently expects. See these 
from arch/powerpc/kernel/pci_dn.c:

struct pci_dn *pci_add_device_node_info(struct pci_controller *hose,
                                        struct device_node *dn)
        const __be32 *type = of_get_property(dn, "ibm,pci-config-space-type", 
        /* Extended config space */
        pdn->pci_ext_config_space = (type && of_read_number(type, 1) == 1);

I had to rework Alexey's "spapr_pci: Create PCI-express root bus  by default"
patch to be able to see the extended config space of a vfio-pci device:

--- a/hw/ppc/spapr_pci.c
+++ b/hw/ppc/spapr_pci.c
@@ -1052,6 +1052,9 @@ static int spapr_populate_pci_child_dt(PCIDevice *dev, 
void *fdt, int offset,
     _FDT(fdt_setprop(fdt, offset, "reg", (uint8_t *)rp.reg, rp.reg_len));
     _FDT(fdt_setprop(fdt, offset, "assigned-addresses",
                      (uint8_t *)rp.assigned, rp.assigned_len));
+    if (sphb->pcie_root && pci_is_express(dev)) {
+        _FDT(fdt_setprop_cell(fdt, offset, "ibm,pci-config-space-type", 0x1));
+    }
     return 0;

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