Re: [PATCH v2] firmware: qemu_fw_cfg.c: hold ACPI global lock during device access

["Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, Mar 17, 2016 at 09:33:40AM -0400, Gabriel L. Somlo wrote:
> On Wed, Mar 16, 2016 at 06:57:01PM +0200, Michael S. Tsirkin wrote:
> > On Tue, Mar 08, 2016 at 01:30:50PM -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-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > > Signed-off-by: Gabriel Somlo <somlo-D+Gtc/HYRWM@public.gmane.org>
> > 
> > So this patch makes sense of course.
> > 
> > 
> > Given the recent discussion on QEMU mailing list,
> > I think there is an additional patch that we need:
> > filter the files exposed to userspace by "opt/" prefix.
> > 
> > This will ensure that we can change all other fw cfg files
> > at will without breaking guest scripts.
> > 
> > Gabriel, could you code this up? Or do you see a
> > pressing need to expose internal QEMU registers to
> > userspace?
> 
> I'd be happy to update the docs to (better) emphasisze that:

Well my experience shows people do not read the docs.
And really, good interfaces should be self-documenting.

>         1 the only way to guarantee any particular item shows up in
>           guest-side fw_cfg sysfs is manually putting it there via the
>           host-side command line
> 
>                 - and BTW, unless you prefixed it with "opt/..." you
>                   are off the reservation, and it might collide with
>                   qemu->firmware communications.
> 
>         2 anything one didn't place there themselves via the qemu
>           command line is informational only, might change or go away
>           at any time, and developing expectations about it based on
>           past observation is done at the observer's own risk.
> 
> While I don't *personally* care about items outside of "opt/", I'm a bit
> uncomfortable actively *hiding* them from userspace -- I could easily
> imagine the ability to see (read-only) fw_cfg content from userspace
> being a handy debugging/troubleshooting tool. It's back to separating
> between mechanism and policy: hiding things from userspace would IMHO
> fall into the policy enforcement side of things, and I'm still unclear
> about the failure scenario we'd be trying to prevent, and its likelihood.
> 
> Thanks,
> --Gabriel

Mostly, we can change internal qemu/firmware interfaces
as long as we verify that firmware that ships with QEMU
does not rely on them.

I'm fine with exposing stuff for debugging purposes
but I would like a cleaner separation between the two,
and self-documenting interfaces.
How about:
	- place everything that is under "opt/" in e.g. "supported"
	  directory, or at root
	- place everything that is not under "opt/" in e.g. "unsupported"
	  directory

Abstracting hardware is what OS is all about, this is not policy.

> > > ---
> > > 
> > > Changes since v1:
> > > 	- no more "#ifdef CONFIG_ACPI"; instead we proceed if
> > > 	  acpi_acquire_global_lock() returns either OK or NOT_CONFIGURED,
> > > 	  and only throw a warning/error message otherwise.
> > > 
> > > 	- didn't get any *negative* feedback from the QEMU crowd, so
> > > 	  this is now a bona-fide "please apply this", rather than just
> > > 	  an RFC :)
> > > 
> > > 	- tested on ACPI-enabled x86_64, and acpi_less ARM (32 and 64 bit)
> > > 	  QEMU VMs (I don't have handy access to an ACPI-enabled ARM VM)
> > > 
> > > Thanks much,
> > >   --Gabriel
> > > 
> > >  drivers/firmware/qemu_fw_cfg.c | 16 ++++++++++++++++
> > >  1 file changed, 16 insertions(+)
> > > 
> > > diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> > > index 7bba76c..a44dc32 100644
> > > --- a/drivers/firmware/qemu_fw_cfg.c
> > > +++ b/drivers/firmware/qemu_fw_cfg.c
> > > @@ -77,12 +77,28 @@ 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)
> > >  {
> > > +	u32 glk;
> > > +	acpi_status status;
> > > +
> > > +	/* If we have ACPI, ensure mutual exclusion against any potential
> > > +	 * device access by the firmware, e.g. via AML methods:
> > > +	 */
> > > +	status = acpi_acquire_global_lock(ACPI_WAIT_FOREVER, &glk);
> > > +	if (ACPI_FAILURE(status) && status != AE_NOT_CONFIGURED) {
> > > +		/* Should never get here */
> > > +		WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
> > > +		memset(buf, 0, count);
> > > +		return;
> > > +	}
> > > +
> > >  	mutex_lock(&fw_cfg_dev_lock);
> > >  	iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
> > >  	while (pos-- > 0)
> > >  		ioread8(fw_cfg_reg_data);
> > >  	ioread8_rep(fw_cfg_reg_data, buf, count);
> > >  	mutex_unlock(&fw_cfg_dev_lock);
> > > +
> > > +	acpi_release_global_lock(glk);
> > >  }
> > >  
> > >  /* clean up fw_cfg device i/o */
> > > -- 
> > > 2.4.3