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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Oct 05, 2015 at 08:43:46AM -0400, Gabriel L. Somlo wrote:
> On Mon, Oct 05, 2015 at 01:23:33PM +0100, Mark Rutland wrote:
> > On Mon, Oct 05, 2015 at 01:48:52PM +0200, Paolo Bonzini wrote:
> > > 
> > > 
> > > On 05/10/2015 12:00, Mark Rutland wrote:
> > > > Some of the keys in the example look like they'd come from other sources
> > > > (e.g. the *-tables entries), while others look like kernel/bootloader
> > > > configuration options (e.g. etc/boot-fail-wait, bootorder) -- I'm
> > > > concerned about redundancy here.
> > > 
> > > The redundancy is because the firmware and the bootloader actually
> > > _consume_ these fw_cfg strings to produce the others (the ACPI tables,
> > > the kernel configuration options).
> > > 
> > > On the other hand, hiding some strings just because they ought to have
> > > been consumed already makes little sense.
> > 
> > Sure. However, I'm concerned that providing redundant interfaces for
> > those could lead to people grabbing information from here (because it's
> > convenient) rather than the existing canonical locations, which means we
> > get more software that works on fewer systems for no good reason.
> > 
> > What I couldn't figure out was what _additional_ information this
> > provided; it looked like a mixed bag of details we could already get
> > from disparate sources. If that's all it does, then it seems to me like
> > it doesn't add any benefit and potentially makes things worse.
> > 
> > So what do we get from this interface that we cannot get elsewhere, and
> > why is this the best way of exposing it?
> 
> Starting with qemu 2.4, it is possible to insert arbitrary named
> blobs into fw_cfg from the qemu command line. *Those* entries
> might be interesting to userspace, which is why it might be handy
> to access to fw_cfg blobs in general.

So this is a mechanism to pass arbitrary key:value pairs to a guest
userspace? What would those be used for, and why would this be the
correct location for that?

How do we avoid clashes between user-selected names and those we need to
pass actual FW data?

Mark.