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Re: [Qemu-devel] [PATCH v3 0/4] SysFS driver for QEMU fw_cfg device
From: |
Gabriel L. Somlo |
Subject: |
Re: [Qemu-devel] [PATCH v3 0/4] SysFS driver for QEMU fw_cfg device |
Date: |
Mon, 5 Oct 2015 09:13:22 -0400 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Mon, Oct 05, 2015 at 01:50:47PM +0100, Peter Maydell wrote:
> On 5 October 2015 at 13:40, Gabriel L. Somlo <address@hidden> 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
[Qemu-devel] [PATCH v3 4/4] firmware: create directory hierarchy for sysfs fw_cfg entries, Gabriel L. Somlo, 2015/10/08
[Qemu-devel] [PATCH v3 3/4] kobject: export kset_find_obj() for module use, Gabriel L. Somlo, 2015/10/08