linux-kernel - Re: [PATCH] iommu/vt-d: Handle hotplug devices' default identity mapping setting

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Date:   Sun, 24 Feb 2019 16:51:06 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     James Dong <xmdong@...gle.com>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jis Ben <jisben@...gle.com>
Subject: Re: [PATCH] iommu/vt-d: Handle hotplug devices' default identity
 mapping setting

Hi James,

On 2/22/19 3:36 PM, James Dong wrote:
> Baolu:
> 
> Sorry that my last reply email seems not text format. Resend it now.
> 
> Thanks for your comments and your patch. Please find below our responses to
> each of your comments:
> 
>> What does "I/O operation won't work" exactly mean here? Do you see any
>> IOMMU fault message? Or, something doesn't work as expected?
> 
> Yes, DMAR fault messages as following came out:
> [  354.939896] DMAR: DMAR:[DMA Read] Request device [03:00.1]fault addr 1fdfe80000
> [  354.939896] DMAR:[fault reason 02] Present bit in context entry is clear
> 
> 
>> Do you mind checking this?
>>
>> index 6ecdcf8fc8c0..f62f30bc1339 100644
>> --- a/drivers/iommu/intel-iommu.c
>> +++ b/drivers/iommu/intel-iommu.c
>> @@ -2632,6 +2632,9 @@ static struct dmar_domain
>> *find_or_alloc_domain(struct device *dev, int gaw)
>>                           goto out;
>>           }
>>
>> +       if (!iommu_should_identity_map(dev, 0))
>> +               return si_domain;
>> +
>>           /* Allocate and initialize new domain for the device */
>>           domain = alloc_domain(0);
>>           if (!domain)
> 
> Tried this patch, and the same DMAR fault message came out.

Thank you!

> 
> Guess it is because of the iommu code path for hotplug devices. If a hotplug
> device is rescanned after removal, iommu_bus_notifier will be called as part
> of the notifier chains to handle BUS_NOTIFY_ADD_DEVICE event. Along the code
> path, intel_iommu_ops->add_device() created an iommu group for this hotplug
> device, but failed to create an iommu domain because of the default domain
> type IOMMU_DOMAIN_IDENTITY imposed by current IOMMU command line option got
> declined by intel_iommu_ops->domain_alloc().

The Intel IOMMU driver hasn't switched to default domain yet although
it's in the pipe line. So, there should be no domain allocated when a
group is allocated for the device.

The problem is we need to check whether a hot-added device requires
identity map instead of allocating a normal domain blindly.

> 
> Since si_domain is type of "struct dmar_domain", which is platform dependent,
> it is hard to make this change in intel_iommu_ops->domain_alloc().
> 
> In your patch, function find_or_alloc_domain() is not in the code path of
> BUS_NOTIFY_ADD_DEVICE event notifier chain.
> 
> Please let us know if your have more concerns and suggestions.

Can you please try the patch attached? I think this is a generic issue
as I described in the commit message.

Best regards,
Lu Baolu

View attachment "0001-iommu-vt-d-Check-identity-map-for-hot-added-devices.patch" of type "text/x-patch" (2586 bytes)