linux-kernel - Re: [PATCH v3 0/4] SysFS driver for QEMU fw

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Date:	Mon, 5 Oct 2015 13:56:47 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	"Gabriel L. Somlo" <somlo@....edu>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, gregkh@...uxfoundation.org,
	paul@...an.com, galak@...eaurora.org, will.deacon@....com,
	agross@...eaurora.org, zajec5@...il.com, hanjun.guo@...aro.org,
	catalin.marinas@....com, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernelnewbies@...nelnewbies.org,
	matt.fleming@...el.com, lersek@...hat.com,
	jordan.l.justen@...el.com, mst@...hat.com,
	peter.maydell@...aro.org, leif.lindholm@...aro.org,
	ard.biesheuvel@...aro.org, kraxel@...hat.com, qemu-devel@...gnu.org
Subject: Re: [PATCH v3 0/4] SysFS driver for QEMU fw_cfg device

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.
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