Re: [PATCH] iommu/vt-d: Fix PCI bus rescan device hot add

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

Hi Lu,

On Fri, 14 Jan 2022 11:12:45 +0800, Lu Baolu <baolu.lu@linux.intel.com>
wrote:

> On 1/14/22 11:11 AM, Jacob Pan wrote:
> > On Fri, 14 Jan 2022 08:58:53 +0800, Lu Baolu<baolu.lu@linux.intel.com>
> > wrote:
> >   
> >> Hi Jacob,
> >>
> >> On 1/13/22 9:23 PM, Jacob Pan wrote:  
> >>> During PCI bus rescan, adding new devices involve two notifiers.
> >>> 1. dmar_pci_bus_notifier()
> >>> 2. iommu_bus_notifier()
> >>> The current code sets #1 as low priority (INT_MIN) which resulted in
> >>> #2 being invoked first. The result is that struct device pointer
> >>> cannot be found in DRHD search for the new device's DMAR/IOMMU.
> >>> Subsequently, the device is put under the "catch-all" IOMMU instead
> >>> of the correct one.
> >>>
> >>> This could cause system hang when device TLB invalidation is sent to
> >>> the wrong IOMMU. Invalidation timeout error or hard lockup can be
> >>> observed.
> >>>
> >>> This patch fixes the issue by setting a higher priority for
> >>> dmar_pci_bus_notifier. DRHD search for a new device will find the
> >>> correct IOMMU.
> >>>
> >>> Fixes: 59ce0515cdaf ("iommu/vt-d: Update DRHD/RMRR/ATSR device scope")
> >>> Reported-by: Zhang, Bernice<bernice.zhang@intel.com>
> >>> Signed-off-by: Jacob Pan<jacob.jun.pan@linux.intel.com>
> >>> ---
> >>>    drivers/iommu/intel/dmar.c | 2 +-
> >>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> >>> index 915bff76fe96..5d07e5b89c2e 100644
> >>> --- a/drivers/iommu/intel/dmar.c
> >>> +++ b/drivers/iommu/intel/dmar.c
> >>> @@ -385,7 +385,7 @@ static int dmar_pci_bus_notifier(struct
> >>> notifier_block *nb,
> >>>    static struct notifier_block dmar_pci_bus_nb = {
> >>>    	.notifier_call = dmar_pci_bus_notifier,
> >>> -	.priority = INT_MIN,
> >>> +	.priority = INT_MAX,
> >>>    };
> >>>    
> >>>    static struct dmar_drhd_unit *
> >>>      
> >> Nice catch! dmar_pci_bus_add_dev() should take place*before*
> >> iommu_probe_device(). This change enforces this with a higher notifier
> >> priority for dmar callback.
> >>
> >> Comparably, dmar_pci_bus_del_dev() should take place*after*
> >> iommu_release_device(). Perhaps we can use two notifiers, one for
> >> ADD_DEVICE (with .priority=INT_MAX) and the other for REMOVE_DEVICE
> >> (with .priority=INT_MIN)?
> >>  
> > Since device_to_iommu() lookup in intel_iommu_release_device() only
> > checks if device is under "an" IOMMU, not "the" IOMMU. Then the remove
> > path order is not needed, right?
> > 
> > I know this is not robust, but having so many notifiers with implicit
> > priority is not clean either.
> > 
> > Perhaps, we should have explicit priority defined around iommu_bus
> > notifier? i.e.
> > 
> > @@ -1841,6 +1841,7 @@ static int iommu_bus_init(struct bus_type *bus,
> > const struct iommu_ops *ops) return -ENOMEM;
> >          nb->notifier_call = iommu_bus_notifier;
> >                         
> > +       nb->priority = IOMMU_BUS_NOTIFY_PRIORITY;
> >                         
> > 
> >   static struct notifier_block dmar_pci_bus_add_nb = {
> >          .notifier_call = dmar_pci_bus_notifier,
> > -       .priority = INT_MIN,
> > +       .priority = IOMMU_BUS_NOTIFY_PRIORITY + 1,
> >   };
> > 
> >   static struct notifier_block dmar_pci_bus_remove_nb = {
> >          .notifier_call = dmar_pci_bus_notifier,
> > -       .priority = INT_MIN,
> > +       .priority = IOMMU_BUS_NOTIFY_PRIORITY - 1,
> >   };  
> 
> IOMMU_BUS_NOTIFY_PRIORITY by default is 0. So you can simply use 1 and
> -1? Adding a comment around it will be helpful.
> 
Yeah, I will add comment.


Thanks,

Jacob