Re: [PATCH v7 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

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

On 9/30/26 2:57 PM, Alex Williamson wrote:
> On Tue, 22 Sep 2026 15:51:39 -0400
> Omar Elghoul <oelghoul@linux.ibm.com> wrote:
> 
>> Introduce the function zpci_fmb_reenable_device() that checks the state
>> of function measurement and ensures it is enabled. Reset the counters to
>> zero, disable, and re-enable the FMB if it was already enabled. Call
>> this function from zpci_reenable_device().
>>
>> Don't free the FMB buffer during disabling and reuse it when re-enabling
>> measurement. Instead, free the buffer upon device teardown, allowing the
>> same buffer to be reused in the enable path and add the bit fmb_enabled
>> to struct zpci_dev. Audit the only consumer of zdev->fmb and update it
>> to reflect the change in semantics.
>>
>> Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
>> ---
>>   arch/s390/include/asm/pci.h |  2 +
>>   arch/s390/pci/pci.c         | 77 +++++++++++++++++++++++++++++--------
>>   arch/s390/pci/pci_debug.c   |  2 +-
>>   3 files changed, 63 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
>> index 88a125b92bdd..b8162f7a8968 100644
>> --- a/arch/s390/include/asm/pci.h
>> +++ b/arch/s390/include/asm/pci.h
>> @@ -175,6 +175,7 @@ struct zpci_dev {
>>   	u8		util_str_avail	: 1;
>>   	u8		tid_avail	: 1;
>>   	u8		rtr_avail	: 1; /* Relaxed translation allowed */
>> +	u8		fmb_enabled	: 1;
> 
> This appears safe, fmb_enabled seems to be the only runtime mutated
> bitfield in this set, but it might be a good idea to flag it as such or
> use a separate storage unit to avoid a tearing hazard in the future.

Great catch, I completely missed that. It makes sense to just move it
into a separate bool since it's the only bit that isn't representing
the existence of some feature/capability, thanks.

> Thanks,
> 
> Alex