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[Qemu-devel] [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock

From: Gabriel Somlo
Subject: [Qemu-devel] [RFC PATCH] firmware: qemu_fw_cfg.c: hold ACPI global lock during device access
Date: Mon, 7 Mar 2016 18:55:43 -0500
User-agent: Mutt/1.5.24 (2015-08-30)

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

Suggested-by: Michael S. Tsirkin <address@hidden>
Signed-off-by: Gabriel Somlo <address@hidden>

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,

 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)
+       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;
+       }
        iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
        while (pos-- > 0)
        ioread8_rep(fw_cfg_reg_data, buf, count);
+       acpi_release_global_lock(glk);
 /* clean up fw_cfg device i/o */

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