Re: [PATCH v3 03/24] PCI: Require Live Update preserved devices are in singleton iommu_groups

[Yi Liu <yi.l.liu@intel.com>]

On 3/25/26 02:00, David Matlack wrote:
> On 2026-03-24 09:07 PM, Yi Liu wrote:
>> On 3/24/26 07:57, David Matlack wrote:
>>> Require that Live Update preserved devices are in singleton iommu_groups
>>> during preservation (outgoing kernel) and retrieval (incoming kernel).
>>>
>>> PCI devices preserved across Live Update will be allowed to perform
>>> memory transactions throughout the Live Update. Thus IOMMU groups for
>>> preserved devices must remain fixed. Since all current use cases for
>>> Live Update are for PCI devices in singleton iommu_groups, require that
>>> as a starting point. This avoids the complexity of needing to enforce
>>> arbitrary iommu_group topologies while still allowing all current use
>>> cases.
>>>
>>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>>> Signed-off-by: David Matlack <dmatlack@google.com>
>>> ---
>>>    drivers/pci/liveupdate.c | 34 +++++++++++++++++++++++++++++++++-
>>>    1 file changed, 33 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
>>> index bec7b3500057..a3dbe06650ff 100644
>>> --- a/drivers/pci/liveupdate.c
>>> +++ b/drivers/pci/liveupdate.c
>>> @@ -75,6 +75,8 @@
>>>     *
>>>     *  * The device must not be a Physical Function (PF).
>>>     *
>>> + *  * The device must be the only device in its IOMMU group.
>>> + *
>>>     * Preservation Behavior
>>>     * =====================
>>>     *
>>> @@ -105,6 +107,7 @@
>>>    #include <linux/bsearch.h>
>>>    #include <linux/io.h>
>>> +#include <linux/iommu.h>
>>>    #include <linux/kexec_handover.h>
>>>    #include <linux/kho/abi/pci.h>
>>>    #include <linux/liveupdate.h>
>>> @@ -222,6 +225,31 @@ static void pci_ser_delete(struct pci_ser *ser, struct pci_dev *dev)
>>>    	ser->nr_devices--;
>>>    }
>>> +static int count_devices(struct device *dev, void *__nr_devices)
>>> +{
>>> +	(*(int *)__nr_devices)++;
>>> +	return 0;
>>> +}
>>> +
>>
>> there was a related discussion on the singleton group check. have you
>> considered the device_group_immutable_singleton() in below link?
>>
>> https://lore.kernel.org/linux-iommu/20220421052121.3464100-4-baolu.lu@linux.intel.com/
> 
> Thanks for the link.
> 
> Based on the discussion in the follow-up threads, I think the only check
> in that function that is needed on top of what is in this patch to
> ensure group immutability is this one:
> 
> 	/*
> 	 * The device could be considered to be fully isolated if
> 	 * all devices on the path from the device to the host-PCI
> 	 * bridge are protected from peer-to-peer DMA by ACS.
> 	 */
> 	if (!pci_acs_path_enabled(pdev, NULL, REQ_ACS_FLAGS))
> 		return false;
> 
> However, this would restrict Live Update support to only device
> topologies that have these flags enabled. I am not yet sure if this
> would be overly restrictive for the scenarios we care about supporting.

yes. It's a bit different from that thread in which not only require
singleton group but also need to be immutable.

> An alternative way to ensure immutability would be to block adding
> devices at probe time. i.e. Fail pci_device_group() if the device being
> added has liveupdate_incoming=True, or if the group already contains a
> device with liveupdate_{incoming,outgoing}=True. We would still need the
> check in pci_liveupdate_preserve() to pretect against setting
> liveupdate_outgoing=True on a device in a multi-device group.

this looks good to me. But you'll disallow hotplug-in during liveupdate.
not sure about if any decision w.r.t. hotplug. is it acceptable?

BTW. A question not specific to this patch. If failure happens after
executing kexec, is there any chance to fallback to the prior kernel?

Regards,
Yi Liu