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

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Date:   Thu, 13 Jan 2022 19:11:22 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH] iommu/vt-d: Fix PCI bus rescan device hot add

Hi BaoLu,

On Fri, 14 Jan 2022 08:58:53 +0800, Lu Baolu <baolu.lu@...ux.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@...el.com>
> > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.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,                       
 };   
               

> Best regards,
> baolu


Thanks,

Jacob