linux-kernel - Re: [PATCH v2 1/2] PCI: Allow per function PCI slots

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Message-ID: <aa0214e8-31be-2b21-c8af-b7831efd60a7@loongson.cn>
Date: Tue, 28 Oct 2025 09:11:56 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Niklas Schnelle <schnelle@...ux.ibm.com>, Farhan Ali
 <alifm@...ux.ibm.com>, linux-s390@...r.kernel.org,
 linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: helgaas@...nel.org, mjrosato@...ux.ibm.com, bblock@...ux.ibm.com,
 agordeev@...ux.ibm.com, gor@...ux.ibm.com, hca@...ux.ibm.com,
 stable@...r.kernel.org, Tianrui Zhao <zhaotianrui@...ngson.cn>,
 Huacai Chen <chenhuacai@...nel.org>
Subject: Re: [PATCH v2 1/2] PCI: Allow per function PCI slots



On 2025/10/27 下午8:29, Niklas Schnelle wrote:
> On Wed, 2025-10-22 at 14:24 -0700, Farhan Ali wrote:
>> On s390 systems, which use a machine level hypervisor, PCI devices are
>> always accessed through a form of PCI pass-through which fundamentally
>> operates on a per PCI function granularity. This is also reflected in the
>> s390 PCI hotplug driver which creates hotplug slots for individual PCI
>> functions. Its reset_slot() function, which is a wrapper for
>> zpci_hot_reset_device(), thus also resets individual functions.
>>
>> Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
>> to multifunction devices. This approach worked fine on s390 systems that
>> only exposed virtual functions as individual PCI domains to the operating
>> system.  Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> s390 supports exposing the topology of multifunction PCI devices by
>> grouping them in a shared PCI domain. When attempting to reset a function
>> through the hotplug driver, the shared slot assignment causes the wrong
>> function to be reset instead of the intended one. It also leaks memory as
>> we do create a pci_slot object for the function, but don't correctly free
>> it in pci_slot_release().
>>
>> Add a flag for struct pci_slot to allow per function PCI slots for
>> functions managed through a hypervisor, which exposes individual PCI
>> functions while retaining the topology.
> 
> I wonder if LoongArch which now also does per PCI function pass-through
> might need this too. Adding their KVM maintainers.

Hi Niklas,

Thanks for your reminder. Yes, LoongArch do per PCI function 
pass-throught. In theory, function pci_slot_enabled_per_func() should 
return true on LoongArch also. Only that now IOMMU driver is not merged, 
there is no way to test it, however we will write down this as a note 
inside about this issue and verify it once IOMMU driver is merged.


Regards
Bibo Mao
> 
>>
>> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> Cc: stable@...r.kernel.org
>> Suggested-by: Niklas Schnelle <schnelle@...ux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@...ux.ibm.com>
>> ---
>>   drivers/pci/pci.c   |  5 +++--
>>   drivers/pci/slot.c  | 25 ++++++++++++++++++++++---
>>   include/linux/pci.h |  1 +
>>   3 files changed, 26 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index b14dd064006c..36ee38e0d817 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -4980,8 +4980,9 @@ static int pci_reset_hotplug_slot(struct hotplug_slot *hotplug, bool probe)
>>   
>>   static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
>>   {
>> -	if (dev->multifunction || dev->subordinate || !dev->slot ||
>> -	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
>> +	if (dev->subordinate || !dev->slot ||
>> +	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
>> +	    (dev->multifunction && !dev->slot->per_func_slot))
>>   		return -ENOTTY;
>>   
>>   	return pci_reset_hotplug_slot(dev->slot->hotplug, probe);
>> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
>> index 50fb3eb595fe..ed10fa3ae727 100644
>> --- a/drivers/pci/slot.c
>> +++ b/drivers/pci/slot.c
>> @@ -63,6 +63,22 @@ static ssize_t cur_speed_read_file(struct pci_slot *slot, char *buf)
>>   	return bus_speed_read(slot->bus->cur_bus_speed, buf);
>>   }
>>   
>> +static bool pci_dev_matches_slot(struct pci_dev *dev, struct pci_slot *slot)
>> +{
>> +	if (slot->per_func_slot)
>> +		return dev->devfn == slot->number;
>> +
>> +	return PCI_SLOT(dev->devfn) == slot->number;
>> +}
>> +
>> +static bool pci_slot_enabled_per_func(void)
>> +{
>> +	if (IS_ENABLED(CONFIG_S390))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
> --- snip ---
>>   
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index d1fdf81fbe1e..6ad194597ab5 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -78,6 +78,7 @@ struct pci_slot {
>>   	struct list_head	list;		/* Node in list of slots */
>>   	struct hotplug_slot	*hotplug;	/* Hotplug info (move here) */
>>   	unsigned char		number;		/* PCI_SLOT(pci_dev->devfn) */
>> +	unsigned int		per_func_slot:1; /* Allow per function slot */
>>   	struct kobject		kobj;
>>   };
>>   
> 
> Reviewed-by: Niklas Schnelle <schnelle@...ux.ibm.com>
>