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

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Date:   Fri, 14 Jan 2022 11:12:45 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com, 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>
Subject: Re: [PATCH] iommu/vt-d: Fix PCI bus rescan device hot add

On 1/14/22 11:11 AM, Jacob Pan wrote:
> 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,
>   };

IOMMU_BUS_NOTIFY_PRIORITY by default is 0. So you can simply use 1 and
-1? Adding a comment around it will be helpful.

Best regards,
baolu