linux-kernel - Re: [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock during device access

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Date:	Tue, 8 Mar 2016 11:21:37 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Gabriel Somlo <somlo@....edu>, 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: [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock
 during device access

On Mon, Mar 07, 2016 at 04:02:15PM -0800, Greg KH wrote:
> On Mon, Mar 07, 2016 at 06:55:43PM -0500, Gabriel Somlo wrote:
> > Allowing for the future possibility of implementing AML-based
> > (i.e., firmware-triggered) access to the QEMU fw_cfg device,
> > acquire the global ACPI lock when accessing the device on behalf
> > of the guest-side sysfs driver, to prevent any potential race
> > conditions.
> > 
> > Suggested-by: Michael S. Tsirkin <mst@...hat.com>
> > Signed-off-by: Gabriel Somlo <somlo@....edu>

ACK, except for Gerd's comments below.

> > ---
> > 
> > Turns out, there *is* a way to acquire a global ACPI lock from within
> > a "random" kernel driver after all. Luckily I have a healthy dose of
> > respect for Michael's opinions :) so I kept circling back through
> > existing kernel sources for an example I can use, and I think this
> > might be it.
> > 
> > I'm posting as RFC because I'm not really confident about assessing
> > the likelihood of there ever being a race condition between the kernel
> > fw_cfg sysfs driver and firmware on my own. Obviously Michael has
> > concerns about it, but any additional opinions from the QEMU camp would
> > be much appreciated.
> > 
> > Thanks again,
> >  --Gabriel
> > 
> >  drivers/firmware/qemu_fw_cfg.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> > index 7bba76c..cc4c27a 100644
> > --- a/drivers/firmware/qemu_fw_cfg.c
> > +++ b/drivers/firmware/qemu_fw_cfg.c
> > @@ -77,12 +77,26 @@ static inline u16 fw_cfg_sel_endianness(u16 key)
> >  static inline void fw_cfg_read_blob(u16 key,
> >  				    void *buf, loff_t pos, size_t count)
> >  {
> > +#ifdef CONFIG_ACPI

You should include linux/acpi.h if you use this,
do not rely on some other header to pull this in.

> > +	u32 glk;
> > +	int status;
> > +	status = acpi_acquire_global_lock(ACPI_WAIT_FOREVER, &glk);
> 
> Why not fix up the ACPI api to not require #ifdef here?  It should be
> added to a .h file somewhere instead.

Indeed. In fact include/acpi/platform/aclinux.h already has
this code:

#ifndef CONFIG_ACPI

/* External globals for __KERNEL__, stubs is needed */

#define ACPI_GLOBAL(t,a)
#define ACPI_INIT_GLOBAL(t,a,b)

/* Generating stubs for configurable ACPICA macros */

#define ACPI_NO_MEM_ALLOCATIONS

/* Generating stubs for configurable ACPICA functions */

#define ACPI_NO_ERROR_MESSAGES
#undef ACPI_DEBUG_OUTPUT

/* External interface for __KERNEL__, stub is needed */

#define ACPI_EXTERNAL_RETURN_STATUS(prototype) \
        static ACPI_INLINE prototype {return(AE_NOT_CONFIGURED);}
#define ACPI_EXTERNAL_RETURN_OK(prototype) \
        static ACPI_INLINE prototype {return(AE_OK);}
#define ACPI_EXTERNAL_RETURN_VOID(prototype) \
        static ACPI_INLINE prototype {return;}
#define ACPI_EXTERNAL_RETURN_UINT32(prototype) \
        static ACPI_INLINE prototype {return(0);}
#define ACPI_EXTERNAL_RETURN_PTR(prototype) \
        static ACPI_INLINE prototype {return(NULL);}

#endif                          /* CONFIG_ACPI */


so it's just a question of testing the return code for AE_NOT_CONFIGURED.

-- 
MST