Re: [Qemu-devel] [RFC for-2.10 1/3] pci/pcie: Make a consistent helper for switching PCI/PCIe "hybrid" devices

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Aug 29, 2017 at 11:12:36AM -0300, Eduardo Habkost wrote:
> On Tue, Aug 29, 2017 at 09:42:39PM +1000, David Gibson wrote:
> > On Wed, Apr 26, 2017 at 06:23:58PM +0300, Michael S. Tsirkin wrote:
> > > On Tue, Mar 28, 2017 at 01:16:49PM +1100, David Gibson wrote:
> > > > virtio-pci and XHCI are "hybrid" devices in the sense that they can present
> > > > themselves as either PCIe or plain PCI devices depending on the machine
> > > > and bus they're connected to.
> > > > 
> > > > For virtio-pci to present as PCIe it requires that it's connected to a PCIe
> > > > bus and that it's not a root bus - this is to ensure that the device is
> > > > connected via a PCIe root port or downstream port rather than being a
> > > > integrated endpoint.  Some guests (Windows in particular AIUI) don't really
> > > > cope with PCIe integrated endpoints.
> > > > 
> > > > For XHCI it only checks that the bus is PCIe, but that probably means it
> > > > would cause problems if attached as an integrated devices directly to a
> > > > PCIe root bus.
> > > > 
> > > > This patch makes the test consistent between XHCI and virtio-pci, and
> > > > clarifies things by having them both use a new 'pci_allow_hybrid_pcie()'
> > > > helper which performs the same check as virtio-pci.
> > > > 
> > > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > > ---
> > > >  hw/pci/pci.c           | 7 +++++++
> > > >  hw/usb/hcd-xhci.c      | 2 +-
> > > >  hw/virtio/virtio-pci.c | 3 +--
> > > >  include/hw/pci/pci.h   | 1 +
> > > >  4 files changed, 10 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > > > index bd8043c..779787b 100644
> > > > --- a/hw/pci/pci.c
> > > > +++ b/hw/pci/pci.c
> > > > @@ -390,6 +390,13 @@ bool pci_bus_is_root(PCIBus *bus)
> > > >      return PCI_BUS_GET_CLASS(bus)->is_root(bus);
> > > >  }
> > > >  
> > > > +bool pci_allow_hybrid_pcie(PCIDevice *pci_dev)
> > > > +{
> > > > +    PCIBus *bus = pci_dev->bus;
> > > > +
> > > > +    return pci_bus_is_express(bus) && !pci_bus_is_root(bus);
> > > > +}
> > > > +
> > > >  void pci_bus_new_inplace(PCIBus *bus, size_t bus_size, DeviceState *parent,
> > > >                           const char *name,
> > > >                           MemoryRegion *address_space_mem,
> > > 
> > > I'd prefer pci_allow_hybrid_pci_pcie.
> > 
> > Ok, I've made that change for the next spin (aimed at 2.11, obviously).
> 
> I'm a bit confused by the naming: by looking at the function
> name, I don't know if "allow hybrid" means "this bus+device can
> (also) work as Conventional PCI" or "this bus+device can (also)
> work as PCI Express".

Neither, actually.  It means "should this device, which is capable of
both PCI and PCIe operation, operate as PCIe in this context".  It's
only expected to be called by devices which support both modes of
operation.

I have yet to think of a succinct name which conveys that :(.

> What about just naming it pci_allow_pcie() or
> pci_bus_allow_pcie()?  It looks like the function doesn't care if
> the device is hybrid or PCIe-only: it's only checking if the
> device can work as PCIe on that bus.  It's up to the device to
> decide if it should switch to Conventional PCI or report an error
> if the function returns false.

Hmm.. that would mean changing *every* existing PCIe device to call
this, which I don't think I want to do.

Also it's _not* really saying if a device can operate as PCIe.  AIUI,
a device _can_ operate as PCIe on a root bus (without a port) although
it's unusual.  Integrated PCIe devices would do so, IIUC.  For that
matter I believe current devices which only support PCIe mode will
also operate in PCIe mode on a root bus right now; but doing so
without inserting a root port might make guests unhappy, at least on
PC.

> 
> > 
> > > 
> > > > diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
> > > > index f0af852..a7ff4fd 100644
> > > > --- a/hw/usb/hcd-xhci.c
> > > > +++ b/hw/usb/hcd-xhci.c
> > > > @@ -3619,7 +3619,7 @@ static void usb_xhci_realize(struct PCIDevice *dev, Error **errp)
> > > >                       PCI_BASE_ADDRESS_SPACE_MEMORY|PCI_BASE_ADDRESS_MEM_TYPE_64,
> > > >                       &xhci->mem);
> > > >  
> > > > -    if (pci_bus_is_express(dev->bus) ||
> > > > +    if (pci_allow_hybrid_pcie(dev) ||
> > > >          xhci_get_flag(xhci, XHCI_FLAG_FORCE_PCIE_ENDCAP)) {
> > > >          ret = pcie_endpoint_cap_init(dev, 0xa0);
> > > >          assert(ret >= 0);
> > > 
> > > This seems to change the behaviour for xhci on a root bus - what
> > > am I missing?
> > 
> > Nothing.  I didn't consider the backwards compat implications; I'll
> > fix it for the next spin.
> > 
> 
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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