Re: [Qemu-devel] [RFC 2/7] fw_cfg dma interface

[Laszlo Ersek]

On 07/23/15 15:35, Peter Maydell wrote:
> On 23 July 2015 at 14:13, Laszlo Ersek <address@hidden> wrote:
>> On 07/22/15 19:18, Kevin O'Connor wrote:
>>> Another possibility would be to place the new fw_cfg dma register
>>> address into a named fw_cfg "file" (eg, "fw_cfg_dma").  The firmware
>>> could then use the existing select/data fw_cfg interface to check if
>>> the new dma interface is available by scanning for that "fw_cfg_dma"
>>> file.  This has the advantage of not requiring a new "magic address",
>>> but has the disadvantage of a more complex probe.
>>
>> I like this one so much that I'm worried I'm missing some details. :)
> 
> This requires the device itself to know its own address, which
> is in QEMU possible but ugly enough to be worth avoiding.
> 
> For ARM MMIO the obvious answer is "the new register should
> just go next to the first one". Does x86 do something that
> means we can't put it somewhere equally straightforward
> or do discovery via whatever x86 uses for discovering MMIO?

I don't know how x86 determines the MMIO mapping. As far as I gather
from the SeaBIOS patches and this QEMU series, 0xfef00000 is a
hand-picked fixed address. (See BIOS_CFG_DMA_ADDR in 7/7.)

0xfef00000 seems to fall right above the 1MB LAPIC range; I guess
there's no conflict with anything else...

Thanks
Laszlo