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Re: [Qemu-devel] fw_cfg specification ?

From: Laszlo Ersek
Subject: Re: [Qemu-devel] fw_cfg specification ?
Date: Wed, 11 Mar 2015 22:30:09 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 03/11/15 21:45, Gabriel L. Somlo wrote:
> On Wed, Mar 11, 2015 at 07:50:40PM +0100, Laszlo Ersek wrote:
>> On 03/11/15 16:27, Gabriel L. Somlo wrote:
>>> Hi,
>>> I'm looking for the closest thing to an official spec for qemu's
>>> fw_cfg device, and so far I have found this:
>>> http://lists.gnu.org/archive/html/qemu-devel/2011-04/msg00238.html
>>> but it apparently never got committed to qemu (any idea why not?).
>> Must have fallen through the cracks. (Just speculating; in April 2011 I
>> had been with RH for less than half a year, and learning Xen. :))
>>> Googling around didn't get me much further than that.
>>> Is there anything better or more up to date floating around out
>>> there somewhere ?
>> I won't say "better", but it is "committed": check
>> "Documentation/devicetree/bindings/arm/fw-cfg.txt" in the kernel tree.
>> It is intentionally vague on the set of keys and fw_cfg files that qemu
>> provides, because that's a moving target. You can only rely on the qemu
>> source for those.
> The fw_cfg interface from the guest's perspective is pretty
> straightforward: select something on the control port, read a blob
> from the data port.
> I was more interested in the "hardware" implementation details,
> and the reasoning behind them.
> For instance, I get there's a total of 0x30 entries
> (FWCfgEntry entries), the last 0x10 of which have 
> "file names" and are referenced from the "FWCfgFiles *files"
> structure.
> I don't quite get the part where
>   struct FWCfgState {
>     ...
>     FWCfgEntry entries[2][FW_CFG_MAX_ENTRY];
>     ...
>   }
> is accessed via 
>   int arch = !!(key & FW_CFG_ARCH_LOCAL);
>   ...
>   s->entries[arch][key].foo = ...
> I.e., what's the significance of arch==0 vs. arch==1 ?

Well, from that aborted :) text file of mine:

> The client writes a 16-bit wide selector to the control port. The most
> significant bit (FW_CFG_ARCH_LOCAL) selects the namespace: 0
> corresponds to "generic" namespace, while 1 corresponds to
> "architecture specific" namespace.

Jordan had also dedicated a paragraph to Bit15 in his patch that you
linked above.

> Also, what's the significance of handling guest-initiated writes to
> the data port ? Could the guest write larger chunks of data than the
> current size of the selected entry ?


Again, from that aborted text file:

> The client code writes a selector (a key) to the control port, and
> then can read the corresponding data (produced by qemu-kvm) via the
> data port, or, if the selected entry is writable, rewrite it. If
> qemu-kvm has associated a callback with the entry: when the entry is
> completely rewritten, qemu-kvm runs the callback. This way the client
> code can exchange data with and even invoke functions in qemu-kvm.

> Would the selected entry's size
> grow accordingly ? Would it shrink if the guest (over)wrote less than
> the current size ?

No. Please see the fw_cfg_write() function. Only the same size can be
written, and the write callback runs when that's completed.

In any case, I'm unaware of *any* instance when an fw_cfg blob is
rewritten by the guest.

In fact, such *write* callbacks are registered in qemu with
fw_cfg_add_callback(), and I can't see any calls to that function. It's
dead code, apparently.

(fw_cfg_add_file_callback() is different; it is much more recent, and it
is for read callbacks.)

> I know it's all going to start making sense eventually, if I stare at
> the source long enough :) I'm just trying to speed up that process as
> much as possible.
> Right now, at a quick glance, qemu writes a bunch of stuff into the
> fw_cfg data structure before starting the guest, and the guest (well,
> mostly the guest's BIOS) does a bunch of things based on the
> "breadcrumbs" it reads from the fw_cfg device.
> But that doesn't seem to be the whole story...

It is pretty close; it's almost the whole story, as far as I'm aware.
What more should an interface called "firmware config" do than, well,
configure the firmware? :)

(Even -kernel / -initrd / -append abuse the original intent of fw_cfg,
because those blobs are huge, and not configuration-like at all. But
they got popular and now we're stuck with them; higher level tools
depend on them heavily.)

There is though a bit more to the story: the (recent) read callbacks.
Qemu sometimes decides to re-generate a "bunch of stuff" in the fw_cfg
blobs. See the commit message here:


(The edk2 patch visible in that commit has been completely reimplemented
since, but the qemu description in the commit *message* remains valid.)

>> If you have a ton of time, you could try documenting fw_cfg yourself,
>> but as I said, it's a moving target, so the description would either
>> become stale quickly, or require people to keep it in sync with the
>> source all the time. Updating documentation sucks *hard*.
> Once I start getting what's going on (e.g., w.r.t. the questions above)
> I wouldn't mind just adding *comments* to the source, for the next guy
> who, like me, is trying to get the lay of the land, but I'm not there
> yet...

I believe that would be useful, yes.


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