linux-kernel - Re: [PATCH v6 0/4] SysFS driver for QEMU fw

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Message-ID: <20151219151006.GA11545@GLSMBP.INI.CMU.EDU>
Date:	Sat, 19 Dec 2015 10:10:07 -0500
From:	"Gabriel L. Somlo" <somlo@....edu>
To:	Pavel Machek <pavel@....cz>
Cc:	gregkh@...uxfoundation.org, robh+dt@...nel.org, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, arnd@...db.de, lersek@...hat.com,
	ralf@...ux-mips.org, rmk+kernel@....linux.org.uk, eric@...olt.net,
	hanjun.guo@...aro.org, zajec5@...il.com, sudeep.holla@....com,
	agross@...eaurora.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	qemu-devel@...gnu.org, jordan.l.justen@...el.com, mst@...hat.com,
	peter.maydell@...aro.org, leif.lindholm@...aro.org,
	ard.biesheuvel@...aro.org, pbonzini@...hat.com, kraxel@...hat.com,
	luto@...capital.net, stefanha@...il.com, revol@...e.fr,
	matt@...eblueprint.co.uk
Subject: Re: [PATCH v6 0/4] SysFS driver for QEMU fw_cfg device

Hi Pavel,

On Sat, Dec 19, 2015 at 10:12:28AM +0100, Pavel Machek wrote:
> On Thu 2015-12-17 11:09:23, Gabriel L. Somlo wrote:
> > ping ?
> > 
> > Also, for the corresponding patch set on the QEMU end of things,
> > ping on http://thread.gmane.org/gmane.comp.emulators.qemu/376321
> 
> I guess missing information is why such access is a good
> idea. Debugging?

Debugging did come up in earlier threads as well. However, personally,
I am interested in a mechanism for the host to pass information to the
guest in a way that's both

	- asynchronous: i.e., host doesn't need to wait for guest to
	  be ready to accept data (e.g. by first starting an agent)

	- out-of-band: don't commandeer guest elements normally visible
	  and available to guest *users* (e.g. kernel command line,
	  mount a floppy/cdrom image, etc).

QEMU now allows arbitrary fw_cfg blobs to be added via the command
line, so it would be nice to make it easy to access them from the
guest OS -- and what's nicer and easier than:

	cat /sys/firmware/qemu-fw-cfg/.../some-blob-name/raw

Thanks,
--Gabriel

> > On Fri, Dec 04, 2015 at 10:29:02AM -0500, Gabriel L. Somlo wrote:
> > > Allow access to QEMU firmware blobs, passed into the guest VM via
> > > the fw_cfg device, through SysFS entries. Blob meta-data (e.g. name,
> > > size, and fw_cfg key), as well as the raw binary blob data may be
> > > accessed.
> > > 
> > > The SysFS access location is /sys/firmware/qemu_fw_cfg/... and was
> > > selected based on overall similarity to the type of information
> > > exposed under /sys/firmware/dmi/entries/...
> 
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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