Re: [PATCH v2 3/3] s390/pci: Fence FMB enable/disable via sysfs for passthrough devices

[Niklas Schnelle]

On Tue, 2026-05-19 at 18:42 -0400, Omar Elghoul wrote:
> 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@xxxxxxxxxxxxx>
> ---
>  arch/s390/pci/pci_debug.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
> index c7ed7bf254b5..2601614b919b 100644
> --- a/arch/s390/pci/pci_debug.c
> +++ b/arch/s390/pci/pci_debug.c
> @@ -149,9 +149,15 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
>  	if (!zdev)
>  		return 0;
>  
> +	mutex_lock(&zdev->kzdev_lock);
> +	if (zdev->kzdev) {
> +		rc = -EPERM;
> +		goto release_kzdev_and_out;

Nit: "release" to me sounds misleading here since it's not about any of
the things called "release" in the kernel but about unlocking a mutex,
I'd probably go with "out_unlock_kzdev" for the label.

> +	}
> +
>  	rc = kstrtoul_from_user(ubuf, count, 10, &val);
>  	if (rc)
> -		return rc;
> +		goto release_kzdev_and_out;
>  
>  	mutex_lock(&zdev->fmb_lock);
>  	switch (val) {
> @@ -163,6 +169,9 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
>  		break;
>  	}
>  	mutex_unlock(&zdev->fmb_lock);
> +
> +release_kzdev_and_out:
> +	mutex_unlock(&zdev->kzdev_lock);
>  	return rc ? rc : count;
>  }
>  

Can't say I love having to fence the host from being able to disable
the FMB via sysfs, but the potential inconsistencies really aren't
covered by the architecture and I don't see a better way.

So with the nit handled feel free to add:

Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>

Thanks,
Niklas