Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device

From: "Michael S. Tsirkin" <mst@redhat.com>
To: "Gabriel L. Somlo" <somlo@cmu.edu>
Cc: gregkh@linuxfoundation.org, robh+dt@kernel.org,
	pawel.moll@arm.com, mark.rutland@arm.com,
	ijc+devicetree@hellion.org.uk, galak@codeaurora.org,
	arnd@arndb.de, lersek@redhat.com, ralf@linux-mips.org,
	rmk+kernel@arm.linux.org.uk, eric@anholt.net,
	hanjun.guo@linaro.org, zajec5@gmail.com, sudeep.holla@arm.com,
	agross@codeaurora.org, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
	qemu-devel@nongnu.org, imammedo@redhat.com,
	peter.maydell@linaro.org, leif.lindholm@linaro.org,
	ard.biesheuvel@linaro.org, pbonzini@redhat.com,
	kraxel@redhat.com, ehabkost@redhat.com, luto@amacapital.net,
	stefanha@gmail.com, revol@free.fr, matt@codeblueprint.co.uk,
	rth@twiddle.net
Subject: Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device
Date: Tue, 23 Feb 2016 07:07:36 +0200	[thread overview]
Message-ID: <20160223070541-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <20160222202623.GI16357@HEDWIG.INI.CMU.EDU>

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