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

1 files changed, 16 insertions, 0 deletions

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index e4c471413d46..815c4a5cae54 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 */