Re: [PATCH v3 0/4] SysFS driver for QEMU fw_cfg device

[Gabriel L. Somlo]

On Mon, Oct 05, 2015 at 01:50:47PM +0100, Peter Maydell wrote:
> On 5 October 2015 at 13:40, Gabriel L. Somlo <somlo@xxxxxxx> 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.

I don't know of any case where firmware and kernel might race each
other to access fw_cfg.

The issue AFAICT is whether it's safe (future-proof) to rely on
parsing _CRS for the fw_cfg i/o access information, or whether
such logic could be rendered obsolete by potential future updates
to fw_cfg's _CRS. If I "outsource" the fw_cfg_dump_blob_by_key()
functionality entirely to an ACPI method, my kernel driver won't
have to worry about keeping up with said future updates.

On the down-side, that means the kernel driver will be ACPI or
nothing (but I'm OK with that, at my curent level of understanding :)

Thanks,
--Gabriel
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