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@xxxxxxxxxx>
Signed-off-by: Gabriel Somlo <somlo@xxxxxxx>
---
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
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