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

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Date:	Mon, 7 Mar 2016 18:55:43 -0500
From:	Gabriel Somlo <somlo@....edu>
To:	"Michael S. Tsirkin" <mst@...hat.com>, gregkh@...uxfoundation.org,
	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: [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock during
 device access

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>
---

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);
+	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);
+#endif
 }
 
 /* clean up fw_cfg device i/o */
-- 
2.4.3