Date: Mon, 7 Mar 2016 16:02:15 -0800
From: Greg KH <gregkh@linuxfoundation.org>
To: Gabriel Somlo <somlo@cmu.edu>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, 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: [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock
 during device access
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References: <20160307235543.GD2049@HEDWIG.INI.CMU.EDU>
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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@redhat.com>
> Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
> ---
> 
> 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
> +	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.


> +	if (ACPI_FAILURE(status)) {
> +		/* Should never get here */
> +		WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
> +		memset(buf, 0, count);
> +		return;
> +	}
> +#endif
>  	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);
> +#ifdef CONFIG_ACPI
> +	acpi_release_global_lock(glk);

Same here.

thanks,

greg k-h