Re: [Qemu-devel] Re: [PATCH] PCI: Bus number from the bridge, not the device

[Gleb Natapov]

On Sun, Nov 21, 2010 at 01:53:26PM +0200, Michael S. Tsirkin wrote:
> > > The guests.
> > Which one? There are many guests. Your favorite?
> > 
> > > For CLI, we need an easy way to map a device in guest to the
> > > device in qemu and back.
> > Then use eth0, /dev/sdb, or even C:. Your way is not less broken since what
> > you are saying is "lets use name that guest assigned to a device". 
> 
> No I am saying let's use the name that our ACPI tables assigned.
> 
ACPI does not assign any name. In a best case ACPI tables describe resources
used by a device. And not all guests qemu supports has support for ACPI. Qemu
even support machines types that do not support ACPI.
 
> > > 
> > > 
> > > > It looks like you identify yourself with most of
> > > > qemu users, but if most qemu users are like you then qemu has not enough
> > > > users :) Most users that consider themselves to be "advanced" may know
> > > > what eth1 or /dev/sdb means. This doesn't mean we should provide
> > > > "device_del eth1" or "device_add /dev/sdb" command though. 
> > > > 
> > > > More important is that "domain" (encoded as number like you used to)
> > > > and "bus number" has no meaning from inside qemu.
> > > > So while I said many
> > > > times I don't care about exact CLI syntax to much it should make sense
> > > > at least. It can use id to specify PCI bus in CLI like this:
> > > > device_del pci.0:1.1. Or it can even use device id too like this:
> > > > device_del pci.0:ide.0. Or it can use HW topology like in FO device
> > > > path. But doing ah-hoc device enumeration inside qemu and then using it
> > > > for CLI is not it.
> > > > 
> > > > > functionality in the guests.  Qemu is buggy in the moment in that is
> > > > > uses the bus addresses assigned by guest and not the ones in ACPI,
> > > > > but that can be fixed.
> > > > It looks like you confused ACPI _SEG for something it isn't.
> > > 
> > > Maybe I did. This is what linux does:
> > > 
> > > struct pci_bus * __devinit pci_acpi_scan_root(struct acpi_pci_root
> > > *root)
> > > {
> > >         struct acpi_device *device = root->device;
> > >         int domain = root->segment;
> > >         int busnum = root->secondary.start;
> > > 
> > > And I think this is consistent with the spec.
> > > 
> > It means that one domain may include several host bridges.
> > At that level
> > domain is defined as something that have unique name for each device
> > inside it thus no two buses in one segment/domain can have same bus
> > number. This is what PCI spec tells you. 
> 
> And that really is enough for CLI because all we need is locate the
> specific slot in a unique way.
> 
At qemu level we do not have bus numbers. They are assigned by a guest.
So inside a guest domain:bus:slot.func points you to a device, but in
qemu does not enumerate buses.

> > And this further shows that using "domain" as defined by guest is very
> > bad idea. 
> 
> As defined by ACPI, really.
> 
ACPI is a part of a guest software that may not event present in the
guest. How is it relevant?

> > > > ACPI spec
> > > > says that PCI segment group is purely software concept managed by system
> > > > firmware. In fact one segment may include multiple PCI host bridges.
> > > 
> > > It can't I think:
> > Read _BBN definition:
> >  The _BBN object is located under a PCI host bridge and must be unique for
> >  every host bridge within a segment since it is the PCI bus number.
> > 
> > Clearly above speaks about multiple host bridge within a segment.
> 
> Yes, it looks like the firmware spec allows that.
It even have explicit example that shows it.

> 
> > >   Multiple Host Bridges
> > > 
> > >   A platform may have multiple PCI Express or PCI-X host bridges. The base
> > >   address for the
> > >   MMCONFIG space for these host bridges may need to be allocated at
> > >   different locations. In such
> > >   cases, using MCFG table and _CBA method as defined in this section means
> > >   that each of these host
> > >   bridges must be in its own PCI Segment Group.
> > > 
> > This is not from ACPI spec,
> 
> PCI Firmware Specification 3.0
> 
> > but without going to deep into it above
> > paragraph talks about some particular case when each host bridge must
> > be in its own PCI Segment Group with is a definite prove that in other
> > cases multiple host bridges can be in on segment group.
> 
> I stand corrected. I think you are right. But note that if they are,
> they must have distinct bus numbers assigned by ACPI.
ACPI does not assign any numbers. Bios enumerates buses and assign
numbers. ACPI, in a base case, describes what BIOS did to OSPM. Qemu sits
one layer below all this and does not enumerate PC buses. Even if we make
it to do so there is not way to guaranty that guest will enumerate them
in the same order since there is more then one way to do enumeration. I
repeated this numerous times to you already.

> 
> > > 
> > > > _SEG
> > > > is not what OSPM uses to tie HW resource to ACPI resource. It used _CRS
> > > > (Current Resource Settings) for that just like OF. No surprise there.
> > > 
> > > OSPM uses both I think.
> > > 
> > > All I see linux do with CRS is get the bus number range.
> > So lets assume that HW has two PCI host bridges and ACPI has:
> >         Device(PCI0) {
> >             Name (_HID, EisaId ("PNP0A03"))
> >             Name (_SEG, 0x00)
> >         }
> >         Device(PCI1) {
> >             Name (_HID, EisaId ("PNP0A03"))
> >             Name (_SEG, 0x01)
> >         }
> > I.e no _CRS to describe resources. How do you think OSPM knows which of
> > two pci host bridges is PCI0 and which one is PCI1?
> 
> You must be able to uniquely address any bridge using the combination of _SEG
> and _BBN.

No at all. And saying "you must be able" without actually show how
doesn't prove anything. _SEG is relevant only for those host bridges
that support MMCONFIG (not all of them do, and none that qemu support
does yet). _SEG points to specific entry in MCFG table and MCFG entry
holds base address for MMCONFIG space for the bridge (this address
is configured by a guest). This is all _SEG does really, no magic at
all. _BBN returns bus number assigned by the BIOS to host bridge. Nothing
qemu visible again. So _SEG and _BBN gives you two numbers assigned by
a guest FW. Nothing qemu can use to identify a device.

> 
> > > And the spec says, e.g.:
> > > 
> > >     the memory mapped configuration base
> > >   address (always corresponds to bus number 0) for the PCI Segment Group
> > >   of the host bridge is provided by _CBA and the bus range covered by the
> > >   base address is indicated by the corresponding bus range specified in
> > >   _CRS.
> > > 
> > Don't see how it is relevant. And _CBA is defined only for PCI Express. Lets
> > solve the problem for PCI first and then move to PCI Express. Jumping from 
> > one
> > to another destruct us from main discussion.
> 
> I think this is what confuses us.  As long as you are using cf8/cfc there's no
> concept of a domain really.
> Thus:
>       /address@hidden
> 
> is probably enough for BIOS boot because we'll need to make root bus numbers
> unique for legacy guests/option ROMs.  But this is not a hardware requirement
> and might become easier to ignore eith EFI.
> 
You do not need MMCONFIG to have multiple PCI domains. You can have one
configured via standard cf8/cfc and another one on ef8/efc and one more
at mmio fce00000 and you can address all of them:
/address@hidden
/address@hidden
/address@hidden

And each one of those PCI domains can have 256 subbridges.

> > > 
> > > > > 
> > > > > That should be enough for e.g. device_del. We do have the need to
> > > > > describe the topology when we interface with firmware, e.g. to 
> > > > > describe
> > > > > the ACPI tables themselves to qemu (this is what Gleb's patches deal
> > > > > with), but that's probably the only case.
> > > > > 
> > > > Describing HW topology is the only way to unambiguously describe device 
> > > > to
> > > > something or someone outside qemu and have persistent device naming
> > > > between different HW configuration.
> > > 
> > > Not really, since ACPI is a binary blob programmed by qemu.
> > > 
> > APCI is part of the guest, not qemu.
> 
> Yes it runs in the guest but it's generated by qemu. On real hardware,
> it's supplied by the motherboard.
> 
It is not generated by qemu. Parts of it depend on HW and other part depend
on how BIOS configure HW. _BBN for instance is clearly defined to return
address assigned bu the BIOS.


> > Just saying "not really" doesn't
> > prove much. I still haven't seen any proposition from you that actually
> > solve the problem. No, "lets use guest naming" is not it. There is no
> > such thing as "The Guest". 
> > 
> > --
> >                     Gleb.
> 
> I am sorry if I didn't make this clear.  I think we should use the domain:bus
> pair to name the root device. As these are unique and 
> 
You forgot to complete the sentence :) But you made it clear enough and
it is incorrect. domain:bus pair not only not unique they do not exist
in qemu at all and as such can't be used to address device. They are
product of HW enumeration done by a guest OS just like eth0 or C:.

--
                        Gleb.