Re: [Qemu-devel] KVM call minutes 2013-01-29 - Port I/O

["Andreas Färber" <afaerber@suse.de>]

Am 30.01.2013 21:20, schrieb Michael S. Tsirkin:
> On Wed, Jan 30, 2013 at 06:55:47PM +0100, Andreas Färber wrote:
>> Am 30.01.2013 12:48, schrieb Peter Maydell:
>>> On 30 January 2013 11:39, Andreas Färber <afaerber@suse.de> wrote:
>>>> Proposal by hpoussin was to move _list_add() code to ISADevice:
>>>> http://lists.gnu.org/archive/html/qemu-devel/2013-01/msg00508.html
>>>>
>>>> Concerns:
>>>> * PCI devices (VGA, QXL) register I/O ports as well
>>>>   => above patches add dependency on ISABus to machines
>>>>      -> "<benh> no mac ever had one"
>>>>   => PCIDevice shouldn't use ISA API with NULL ISADevice
>>>> * Lack of avi: Who decides about memory API these days?
>>>>
>>>> armbru and agraf concluded that moving this into ISA is wrong.
>>>>
>>>> => I will drop the remaining ioport patches from above series.
>>>>
>>>> Suggestions on how to proceed with tackling the issue are welcome.
>>>
>>> How does this stuff work on real hardware? I would have
>>> expected that a PCI device registering the fact it has
>>> IO ports would have to do so via the PCI controller it
>>> is plugged into...
>>>
>>> My naive don't-know-much-about-portio suggestion is that this
>>> should work the same way as memory regions: each device
>>> provides portio regions,
>>
>> One remark on "same way as memory regions", me not knowing all the gory
>> hardware details myself.
>>
>> PIO often contradicts the normal MemoryRegion usage. I.e., for an MMIO
>> device you would have a continuous region from say 0xa0000000 to
>> 0xa007ffff inclusive and within that region you have some kind of sparse
>> registers. With ISA ports you often have dense overlapping ranges, say,
>> 0x3-0x6 byte-reads foo, while 0x4 word-write does bar.
> 
> Hmm on x86 this is what happens with cf8..cfb range registers for example.
> We plan handle this ATM using memory region priorities.
> Same would work for prep won't it?

Hm, my point was that iiuc a MemoryRegion is per-address-range whereas
for I/O ports we seem to have per-data-width mappings.

Priorities would allow us to say:

0x1    -    0xff  is one region
    0x8-0xab      is a region with higher priority

but fallback for, e.g., word-access at 0xa0 to the lower-priority region
being unsupported today, no? I.e., the region being opaque.

Having said that, for the purposes of this discussion PReP is pretty
much a PC with a PowerPC CPU in it, unlike the modern CHRP machines.

Andreas

>> This is handled by having lists of (offset, length, size, handler)
>> quadruplets and consolidating those into MemoryRegions and aliases (cf.
>> patches) that then have a validation function to check whether a
>> particular access is valid and by whom it should be handled - that's
>> what MemoryRegionPortio[] and similar APIs are good for.
>>
>> So yes, it might be possible to have a device declare its ports at
>> PCIDevice or DeviceState level, but it can't be directly passed through
>> to MemoryRegion API in most cases, or conflicts would arise. At least
>> that was my experience with PReP.

-- 
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg