linux-kernel - Re: [PATCH] thunderbolt: Stop using iommu

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Date:   Wed, 16 Mar 2022 14:49:09 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     andreas.noever@...il.com, michael.jamet@...el.com,
        YehezkelShB@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        mario.limonciello@....com, hch@....de
Subject: Re: [PATCH] thunderbolt: Stop using iommu_present()

On 2022-03-16 12:45, Mika Westerberg wrote:
> Hi Robin,
> 
> On Wed, Mar 16, 2022 at 11:25:51AM +0000, Robin Murphy wrote:
>> Even if an IOMMU might be present for some PCI segment in the system,
>> that doesn't necessarily mean it provides translation for the device
>> we care about. Furthermore, the presence or not of one firmware flag
>> doesn't imply anything about the IOMMU driver's behaviour, which may
>> still depend on other firmware properties and kernel options too. What
>> actually matters is whether an IOMMU is enforcing protection for our
>> device - regardless of whether that stemmed from firmware policy, kernel
>> config, or user control - at the point we need to decide whether to
>> authorise it. We can ascertain that generically by simply looking at
>> whether we're currently attached to a translation domain or not.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>
>> I don't have the means to test this, but I'm at least 80% confident
>> in my unpicking of the structures to retrieve the correct device...
>>
>>   drivers/thunderbolt/domain.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
>> index 7018d959f775..5f5fc5f6a09b 100644
>> --- a/drivers/thunderbolt/domain.c
>> +++ b/drivers/thunderbolt/domain.c
>> @@ -257,13 +257,14 @@ static ssize_t iommu_dma_protection_show(struct device *dev,
>>   					 struct device_attribute *attr,
>>   					 char *buf)
>>   {
>> +	struct tb *tb = container_of(dev, struct tb, dev);
>> +	struct iommu_domain *iod = iommu_get_domain_for_dev(&tb->nhi->pdev->dev);
> 
> I wonder if this is the correct "domain"? I mean it's typically no the
> Thunderbolt controller (here tb->nhi->pdev->dev) that needs the
> protection (although in discrete controllers it does get it too) but
> it's the tunneled PCIe topology that we need to check here.
> 
> For instance in Intel with intergrated Thunderbolt we have topology like
> this:
> 
>    Host bridge
>        |
>        +--- Tunneled PCIe root port #1
>        +--- Tunneled PCIe root port #2
>        +--- Thunderbolt host controller (the NHI above)
>        +--- xHCI
> 
> and In case of discrete controllers it looks like this:
> 
>    Host bridge
>        |
>        +--- PCIe root port #x
>                  |
>                  |
>             PCIe switch upstream port
>                  |
> 	        +--- Tunneled PCIe switch downstream port #1
> 	        +--- Tunneled PCIe switch downstream port #2
>          	+--- Thunderbolt host controller (the NHI above)
>          	+--- xHCI
> 
> What we want is to make sure the Tunneled PCIe ports get the full IOMMU
> protection. In case of the discrete above it is also fine if all the
> devices behind the PCIe root port get the full IOMMU protection. Note in
> the integrated all the devices are "siblings".

Ah, OK, I wasn't aware that the NHI isn't even the right thing in the 
first place :(

Is there an easy way to get from the struct tb to a PCI device 
representing the end of its relevant tunnel, or do we have a circular 
dependency problem where the latter won't appear until we've authorised 
it (and thus the IOMMU layer won't know about it yet either)?

Thanks,
Robin.