Re: KVM call minutes 2013-01-29 - Port I/O

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 30, 2013 at 03:39:34PM -0600, Anthony Liguori wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> writes:
> 
> > On Wed, 2013-01-30 at 07:59 -0600, Anthony Liguori wrote:
> >> An x86 CPU has a MMIO capability that's essentially 65 bits.  Whether
> >> the top bit is set determines whether it's a "PIO" transaction or an
> >> "MMIO" transaction.  A large chunk of that address space is invalid of
> >> course.
> >> 
> >> PCI has a 65 bit address space too.  The 65th bit determines whether
> >> it's an IO transaction or an MMIO transaction.
> >
> > This is somewhat an over simplification since IO and MMIO differs in
> > other ways, such as ordering rules :-) But for the sake of memory
> > regions decoding I suppose it will do.
> >
> >> For architectures that only have a 64-bit address space, what the PCI
> >> controller typically does is pick a 16-bit window within that address
> >> space to map to a PCI address with the 65th bit set.
> >
> > Sort-of yes. The window doesn't have to be 16-bit (we commonly have
> > larger IO space windows on powerpc) and there's a window per host
> > bridge, so there's effectively more than one IO space (as there is more
> > than one PCI MMIO space, with only a window off the CPU space routed to
> > each brigde).
> 
> Ack.
> 
> > Making a hard wired assumption that the PCI (MMIO and IO) space relates
> > directly to the CPU bus space is wrong on pretty much all !x86
> > architectures.
> 
> Ack.
> 
> >
> >  .../...
> >
> > You make it sound like substractive decode is a chipset hack. It's not,
> > it's specified in the PCI spec.
> 
> It's a hack :-)  It's a well specified hack, but it's still a hack.
> 
> >> 1) A chipset will route any non-positively decoded IO transaction (65th
> >>    bit set) to a single end point (usually the ISA-bridge).  Which one it
> >>    chooses is up to the chipset.  This is called subtractive decoding
> >>    because the PCI bus will wait multiple cycles for that device to
> >>    claim the transaction before bouncing it.
> >
> > This is not a chipset matter. It's the ISA bridge itself that does
> > substractive decoding.
> 
> The PCI bus can have one end point that that can be the target for
> subtractive decoding (not hard decoding, subtractive decoding).  IOW,
> you can only have a single ISA Bridge within a single PCI domain.
> 
> You are right--chipset is the wrong word.  I'm used to thinking in terms
> of only a single domain :-)
> 
> > There also exists P2P bridges doing such substractive
> > decoding, this used to be fairly common with transparent bridges used for
> > laptop docking.
> 
> I'm not sure I understand how this would work.  How can two devices on
> the same PCI domain both do subtractive decoding?  Indeed, the PCI spec
> even says:
> 
> "Subtractive decoding can be implemented by only one device on the bus
>  since it accepts all accesses not positively decoded by some other
>  agent."
> 
> >> 2) There are special hacks in most PCI chipsets to route very specific
> >>    addresses ranges to certain devices.  Namely, legacy VGA IO transactions
> >>    go to the first VGA device.  Legacy IDE IO transactions go to the first
> >>    IDE device.  This doesn't need to be programmed in the BARs.  It will
> >>    just happen.
> >
> > This is also mostly not a hack in the chipset. It's a well defined behaviour
> > for legacy devices, sometimes call hard decoding. Of course often those devices
> > are built into the chipset but they don't have to. Plug-in VGA devices will
> > hard decode legacy VGA regions for both IO and MMIO by default (this can be
> > disabled on most of them nowadays) for example. This has nothing to do with
> > the chipset.
> 
> So I understand what you're saying re: PCI because the devices actually
> assert DEVSEL to indicate that they handle the transaction.
> 
> But for PCI-E, doesn't the controller have to expressly identify what
> the target is?  Is this done with the device class?

Well you can have a PCI bridge and a legacy device behind that.
I think real PCI express devices can not be mapped onto legacy address
ranges.


> > There's a specific bit in P2P bridge to control the forwarding of legacy
> > transaction downstream (and VGA palette snoops), this is also fully specified
> > in the PCI spec.
> 
> Ack.
> 
> >
> >> 3) As it turns out, all legacy PIIX3 devices are positively decoded and
> >>    sent to the ISA-bridge (because it's faster this way).
> >
> > Chipsets don't "send to a bridge". It's the bridge itself that
> > decodes.
> 
> With PCI...
> 
> >> Notice the lack of the word "ISA" in all of this other than describing
> >> the PCI class of an end point.
> >
> > ISA is only relevant to the extent that the "legacy" regions of IO space
> > originate from the original ISA addresses of devices (VGA, IDE, etc...)
> > and to the extent that an ISA bus might still be present which will get
> > the transactions that nothing else have decoded in that space.
> 
> Ack.
> 
> >  
> >> So how should this be modeled?
> >> 
> >> On x86, the CPU has a pio address space.  That can propagate down
> >> through the PCI bus which is what we do today.
> >> 
> >> On !x86, the PCI controller ought to setup a MemoryRegion for
> > downstream
> >> PIO that devices can use to register on.
> >> 
> >> We probably need to do something like change the PCI VGA devices to
> >> export a MemoryRegion and allow the PCI controller to device how to
> >> register that as a subregion.
> >
> > The VGA device should just register fixed address port IOs the same way
> > it would register an IO BAR. Essentially, hard coded IO addresses (or
> > memory, VGA does memory too, don't forget that) are equivalent to having
> > an invisible BAR with a fixed value in it.
> 
> Ack.
> 
> >
> > There should be no "global port IO" because that concept is broken on
> > real multi-domain setups. Those "legacy" address ranges are just
> > hard-wired sub regions of the normal PCI space on which the device sits
> > on (unless you start doing real non-PCI ISA x86).
> 
> So, I think what you're suggesting (and I agree with), is that each PCI
> device should export one or more MemoryRegions and indicate what the
> MemoryRegions are for.
> 
> Potential options are:
> 
>  - MMIO BAR
>  - PIO BAR
>  - IDE hard decode
>  - VGA hard decode
>  - subtractive decode
> 
> I'm very much in agreement if that's what you're suggesting.
> 
> Regards,
> 
> Anthony Liguori
> 
> >
> > Cheers,
> > Ben.