[PATCH v1 3/3] s390/pci: Fence FMB enable/disable via sysfs for passthrough devices

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

Introduce a fence over enabling or disabling FMB via sysfs when the zPCI
device is associated with a KVM. This will allow a KVM guest to use FMB
passthrough and avoid the edge-case where the host disables FMB while the
guest is still using it, which may cause partial counter resets and
inconsistent reads which have no parallel in the architecture.

With this patch, the userspace driver, likely QEMU, is still able to enable
or disable the FMB using the VFIO device feature introduced in the previous
patch, effectively securing what is associated with the VM state and
isolating it from other processes on the host.

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 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index c7ed7bf254b5..b28923395d03 100644
--- a/arch/s390/pci/pci_debug.c
+++ b/arch/s390/pci/pci_debug.c
@@ -149,6 +149,9 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 	if (!zdev)
 		return 0;
 
+	if (zdev->kzdev)
+		return -EPERM;
+
 	rc = kstrtoul_from_user(ubuf, count, 10, &val);
 	if (rc)
 		return rc;
-- 
2.52.0