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

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Date:   Fri, 14 Jan 2022 08:58:53 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com, 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

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)?

Best regards,
baolu