[PATCH v5 3/4] s390/pci: Fence FMB enable/disable via debugfs for passthrough devices

[Omar Elghoul <oelghoul@linux.ibm.com>]

Introduce a fence over enabling or disabling FMB via debugfs when the zPCI
device is associated with a KVM. This will prevent processes on the host
from tampering with the FMB while the guest is still using it, which may
cause partial counter resets and inconsistent reads which have no parallel
in the architecture.

For VFIO devices that are not associated with a KVM (i.e., for userspace
drivers other than QEMU), this fence does not take effect.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 arch/s390/pci/pci_debug.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index c7ed7bf254b5..23eb7e72c870 100644
--- a/arch/s390/pci/pci_debug.c
+++ b/arch/s390/pci/pci_debug.c
@@ -153,6 +153,12 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 	if (rc)
 		return rc;
 
+	mutex_lock(&zdev->kzdev_lock);
+	if (zdev->kzdev) {
+		rc = -EPERM;
+		goto out_unlock_kzdev;
+	}
+
 	mutex_lock(&zdev->fmb_lock);
 	switch (val) {
 	case 0:
@@ -163,6 +169,9 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 		break;
 	}
 	mutex_unlock(&zdev->fmb_lock);
+
+out_unlock_kzdev:
+	mutex_unlock(&zdev->kzdev_lock);
 	return rc ? rc : count;
 }
 
-- 
2.54.0