On 5 October 2015 at 13:40, Gabriel L. Somlo [off-list ref] wrote:

In addition, Michael's comment earlier in the thread suggests that
even my current acpi version isn't sufficiently "orthodox" w.r.t.
ACPI, and I should be providing the hardware access routine as
an ACPI/AML routine, to avoid race conditions with the rest of ACPI,
and for encapsulation. I.e. it's even rude to use the fw_cfg node's
ACPI _CRS method (the part where I'd be treating it like a DT stand-in
only to query fw_cfg's hardware specifics).

If you want to try to support "firmware might also be reading
fw_cfg at the same time as the kernel" this is a (painful)
problem regardless of how the kernel figures out whether a
fw_cfg device is present. I had assumed that one of the design
assumptions of this series was that firmware would only
read the fw_cfg before booting the guest kernel and never touch
it afterwards. If it might touch it later then letting the
guest kernel also mess with fw_cfg seems like a really bad idea.

thanks
-- PMM