Re: [PATCH v4 2/4] s390/pci: Preserve FMB state in device re-enablement

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

On 6/22/26 4:22 PM, Alex Williamson wrote:
> On Fri, 12 Jun 2026 14:10:46 -0400
> Omar Elghoul <oelghoul@linux.ibm.com> wrote:
>> +int zpci_fmb_reenable_device(struct zpci_dev *zdev)
>> +{
>> +	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
>> +	struct zpci_fib fib = {0};
>> +	u8 cc, status;
>> +	int rc;
>> +
>> +	lockdep_assert_held(&zdev->fmb_lock);
>> +
>> +	if (!zdev->fmb)
>> +		return zpci_fmb_enable_device(zdev);
>> +
>> +	fib.gd = zdev->gisa;
>> +	cc = zpci_mod_fc(req, &fib, &status); /* Disable function measurement */
>> +
>> +	/* Unlike in zpci_fmb_disable_device(), cc == 3 is not a valid state here
>> +	 * because we are re-enabling function measurement for the same function
>> +	 * handle.
>> +	 */
>> +	if (cc)
>> +		return -EIO;
>> +
>> +	zpci_fmb_clear_iommu_ctrs(zdev);
>> +
>> +	rc = zpci_fmb_do_enable(zdev);
>> +	if (rc) {
>> +		kmem_cache_free(zdev_fmb_cache, zdev->fmb);
>> +		zdev->fmb = NULL;
>> +	}
>> +
>> +	return rc;
>> +}
>> +EXPORT_SYMBOL_GPL(zpci_fmb_reenable_device);
> 
> Why is this exported?  Maybe this is leftover from the previous version
> where it was noted that the API claimed a failure on double enable that
> didn't actually occur because if used reenable.  Now we seem to have
> the alternate inconsistency, that FMB is automatically enabled as part
> of the standard PCI enable path, so the only initial operation
> available to userspace is to first disable FMB, which is an awkward
> API.  The v3 problem could have been solved in documentation, noting
> that a nested enable resets counters[1], while a nested disable
> generates an errno.  Thanks,

Acked in my reply to patch 3/4

> 
> Alex
> 
> [1] Also note the sashiko question whether reenable actually zeros the
> current fmb buffer or only the internal counters
> https://sashiko.dev/#/message/20260612182632.E4EE71F000E9%40smtp.kernel.org

This question may not be applicable because firmware resets them in
practice, but I will discuss it further with my team.

Thanks.