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

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.com>
> Signed-off-by: Gabriel Somlo <somlo@cmu.edu>

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?

> ---
> 
> 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