[libvirt] Proposal PCI/PCIe device placement on PAPR guests
Greg Kurz
groug at kaod.org
Fri Jan 13 08:58:28 UTC 2017
On Fri, 13 Jan 2017 15:48:31 +1100
David Gibson <david at gibson.dropbear.id.au> wrote:
> On Thu, Jan 12, 2017 at 10:09:03AM +0100, Greg Kurz wrote:
> > On Thu, 12 Jan 2017 17:19:40 +1100
> > Alexey Kardashevskiy <aik at ozlabs.ru> 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 <david at gibson.dropbear.id.au> 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 “ibm,pci-con-
> > fig-space-type” property (see Section B.6.5.1.1.1‚ “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.
> >
> > and
> >
> > B.6.5.1.1.1 Properties for Children of PCI Host Bridges
> >
> > “ibm,pci-config-space-type”
> > 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 lines
> > 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", NULL);
> > .
> > .
> > .
> > /* Extended config space */
> > pdn->pci_ext_config_space = (type && of_read_number(type, 1) == 1);
>
> Ok, thanks for the information.
>
> > 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:
>
> Ah! Is there an easy command line way to verify that extended config
> space is accessible?
>
'lspci -vv' prints the capabilities from the config space with their
hex offset between [ ]. Extended config space starts at offset 0x100.
> > --- 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;
> > }
>
> I'm looking at merging the patch below into ppc-for-2.9 shortly. It
> basically combines a revised version of the pcie option part of
> Alexey's patch with your DT patch above. For now it only allows PCIe
> with an explicit option, changing the default waits on more
> investigation of how to not break things with libvirt.
>
> From e14702a0bb76900077d82510280e1a023c384b72 Mon Sep 17 00:00:00 2001
> From: David Gibson <david at gibson.dropbear.id.au>
> Date: Fri, 13 Jan 2017 15:45:30 +1100
> Subject: [PATCH] spapr_pci: Allow PCI-Express devices
>
> Currently the PCI host bridge (PHB) for the pseries machine type always
> creates a plain PCI bus (TYPE_PCI_BUS). This means that qemu won't allow
> PCIe devices to be attached, and consequently, PCIe devices can't be used
> on pseries.
>
Well, PCIe devices can still be attached actually (at least I could attach
a vfio-pci device linked to a PCIe device on the host). But, they appear
as legacy PCI devices in the guest, without the extended config space.
> This limitation isn't inherent in the PAPR specification. In fact, because
> access to the PCI bus is paravirtualized via hypercalls, from the guest
> point of view there is very little difference between a plain PCI and PCIe
> bus under PAPR.
>
> So, to allow PCIe devices, add a "pcie" option to the PHB device, which
> when enabled creates a PCIe bus. In addition, on PCIe devices under such
> a PHB, we publish the device tree flag which indicates that the guest may
> access PCIe extended config space (via RTAS calls which we've already
> implemented).
>
> Note that this doesn't change the default PHB to PCIe. We probably want to
> do that in future, but we need to sort out some possible breakages with
> libvirt and existing users first. This at least lets knowledgeable users
> experiment with PCIe devices in the meantime.
>
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
FWIW,
Reviewed-by: Greg Kurz <groug at kaod.org>
> ---
> hw/ppc/spapr_pci.c | 8 +++++++-
> include/hw/pci-host/spapr.h | 1 +
> 2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> index fd6fc1d..d17df1f 100644
> --- a/hw/ppc/spapr_pci.c
> +++ b/hw/ppc/spapr_pci.c
> @@ -1053,6 +1053,10 @@ static int spapr_populate_pci_child_dt(PCIDevice *dev, void *fdt, int offset,
> _FDT(fdt_setprop(fdt, offset, "assigned-addresses",
> (uint8_t *)rp.assigned, rp.assigned_len));
>
> + if (sphb->pcie && pci_is_express(dev)) {
> + _FDT(fdt_setprop_cell(fdt, offset, "ibm,pci-config-space-type", 0x1));
> + }
> +
> return 0;
> }
>
> @@ -1434,7 +1438,8 @@ static void spapr_phb_realize(DeviceState *dev, Error **errp)
> bus = pci_register_bus(dev, NULL,
> pci_spapr_set_irq, pci_spapr_map_irq, sphb,
> &sphb->memspace, &sphb->iospace,
> - PCI_DEVFN(0, 0), PCI_NUM_PINS, TYPE_PCI_BUS);
> + PCI_DEVFN(0, 0), PCI_NUM_PINS,
> + sphb->pcie ? TYPE_PCIE_BUS : TYPE_PCI_BUS);
> phb->bus = bus;
> qbus_set_hotplug_handler(BUS(phb->bus), DEVICE(sphb), NULL);
>
> @@ -1592,6 +1597,7 @@ static Property spapr_phb_properties[] = {
> DEFINE_PROP_UINT32("numa_node", sPAPRPHBState, numa_node, -1),
> DEFINE_PROP_BOOL("pre-2.8-migration", sPAPRPHBState,
> pre_2_8_migration, false),
> + DEFINE_PROP_BOOL("pcie", sPAPRPHBState, pcie, false),
> DEFINE_PROP_END_OF_LIST(),
> };
>
> diff --git a/include/hw/pci-host/spapr.h b/include/hw/pci-host/spapr.h
> index 092294e..deed2cc 100644
> --- a/include/hw/pci-host/spapr.h
> +++ b/include/hw/pci-host/spapr.h
> @@ -79,6 +79,7 @@ struct sPAPRPHBState {
> uint64_t dma64_win_addr;
>
> uint32_t numa_node;
> + bool pcie;
>
> /* Fields for migration compatibility hacks */
> bool pre_2_8_migration;
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