linux-kernel - Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw

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Date:	Tue, 23 Feb 2016 07:07:36 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Gabriel L. Somlo" <somlo@....edu>
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, imammedo@...hat.com,
	peter.maydell@...aro.org, leif.lindholm@...aro.org,
	ard.biesheuvel@...aro.org, pbonzini@...hat.com, kraxel@...hat.com,
	ehabkost@...hat.com, luto@...capital.net, stefanha@...il.com,
	revol@...e.fr, matt@...eblueprint.co.uk, rth@...ddle.net
Subject: Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's
 fw_cfg device

On Mon, Feb 22, 2016 at 03:26:23PM -0500, Gabriel L. Somlo wrote:
> On Mon, Feb 22, 2016 at 10:14:50PM +0200, Michael S. Tsirkin wrote:
> > On Sun, Feb 21, 2016 at 08:06:17AM -0500, Gabriel L. Somlo wrote:
> > > > > +static void fw_cfg_io_cleanup(void)
> > > > > +{
> > > > > +	if (fw_cfg_is_mmio) {
> > > > > +		iounmap(fw_cfg_dev_base);
> > > > > +		release_mem_region(fw_cfg_p_base, fw_cfg_p_size);
> > > > > +	} else {
> > > > > +		ioport_unmap(fw_cfg_dev_base);
> > > > > +		release_region(fw_cfg_p_base, fw_cfg_p_size);
> > > > > +	}
> > > > > +}
> > > > > +
> > > > > +/* arch-specific ctrl & data register offsets are not available in ACPI, DT */
> > > > 
> > > > So for all arches which support ACPI, I think this driver
> > > > should just rely on ACPI.
> > > 
> > > There was a discussion about that a few versions ago, and IIRC the
> > > conclusion was not to expect the firmware to contend for fw_cfg access
> > > after the guest kernel boots:
> > > 
> > > https://lkml.org/lkml/2015/10/5/283
> > > 
> > 
> > So it looks like NVDIMM at least wants to pass label data to guest -
> > for which fw cfg might be a reasonable choice.
> > 
> > I suspect things changed - fw cfg used to be very slow but we now have
> > DMA interface which makes it useful for a range of applications.

Comment on this? I'm really worried we'll release linux
without a way to access fw cfg from aml.
How about taking acpi lock around all accesses?

> > > (I even had a prototype version doing what you suggested, but per the above
> > > reference decided to drop it -- which IMHO is for the better, since otherwise
> > > I'd have had to ifdef between ACPI and non-ACPI versions of the driver --
> > > see https://lkml.org/lkml/2015/11/4/534 )
> > 
> > I'm not sure why you have these ifdefs - they are on the host, are they
> > not?
> 
> Think of those as "pseudocode" ifdefs, they're there to distinguish
> between AML that would be generated on MMIO vs. IOPORT systems
> (specifically, arm vs. x86, respectively)
> 
> Some of the AML is the same, but obviously the _CRS, and
> OperationRegion + Field are different, and I wanted to point that out
> somehow :)
> 
> Cheers,
> --Gabriel

You can do ifs as well.

-- 
MST