Re: [PATCH v8 3/4] s390/pci: Fence FMB enable/disable via debugfs for passthrough devices

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2026-10-05 at 11:45 -0400, Omar Elghoul wrote:
> 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>
> Reviewed-by: Matthew Rosato <mjrosato@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 44f026ead414..afc52bd5271a 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;
>  }
>  

Looks good, thank!

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>