Re: [Qemu-devel] [RFC] Writeable files in fw_cfg

[David Woodhouse]

On Sun, 2013-01-27 at 16:02 +0000, Blue Swirl wrote:
> On Sun, Jan 27, 2013 at 3:50 PM, David Woodhouse <address@hidden> wrote:
> > On Sun, 2013-01-27 at 15:14 +0000, Blue Swirl wrote:
> >> It looks like this duplicates rom_add_file() and fw_cfg_add_file(), so
> >> I don't see the point.
> >
> > Both of those are read-only, surely? The firmware inside the guest can't
> > use them for non-volatile storage.
> 
> Right. But why do we need PV non-volatile storage, instead of normal flash?

People have looked at adding normal flash to the PC target, but it has a
number of problems. Firstly, it doesn't even *work* with KVM enabled.
But even if we were to somehow fix that, the simple approach of
emulating a BIOS flash is also very suboptimal; it puts the whole of the
firmware *into* the flash and lets it store its non-volatile variables
in the same emulated flash chip. So if you want to update the firmware
(which would normally be just a case of updating the bios.bin file), you
end up having to do each guest system individually *and* the update is
painful because you have to preserve the non-volatile storage while
updating the other parts.

It's much better to have a separate storage for the non-volatile
variables. Yes, we *could* teach it to use a dedicated flash chip for
that rather than using flash for the firmware *and* storage as we would
on real hardware. But if we're going to do something different for the
virtualised case, a writeable fw_cfg file seems a whole lot easier and
saner.

> > It doesn't duplicate fw_cfg_add_file(); it extends it to allow a
> > writeable mode too. fw_cfg_add_file() becomes a simple wrapper around
> > fw_cfg_add_file_writeable(), which actually contains most of the
> > contents of the original function.
> 
> OK, however most of the loading logic is now in loader.c and that
> should not be duplicated either.

I actually got file_get_size() from pc.c, and had marked it with a
horrid C99 comment so we couldn't miss the duplication. I'll sort out
that kind of detail later, if we can reach consensus on the basic
approach.

-- 
dwmw2

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