linux-kernel - Re: [PATCH] iommu: Fallback to default setting when def_domain

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Date:   Tue, 6 Jul 2021 19:02:57 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     Joerg Roedel <joro@...tes.org>, will@...nel.org,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu: Fallback to default setting when def_domain_type()
 callback returns 0

On 2021-07-06 17:21, Kai-Heng Feng wrote:
> On Tue, Jul 6, 2021 at 5:27 PM Robin Murphy <robin.murphy@....com> wrote:
>>
>> On 2021-07-06 07:51, Kai-Heng Feng wrote:
>>> Commit 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted
>>> device into core") not only moved the check for untrusted device to
>>> IOMMU core, it also introduced a behavioral change by returning
>>> def_domain_type() directly without checking its return value. That makes
>>> many devices no longer use the default IOMMU setting.
>>>
>>> So revert back to the old behavior which defaults to
>>> iommu_def_domain_type when driver callback returns 0.
>>>
>>> Fixes: 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted device into core")
>>
>> Are you sure about that? From that same commit:
>>
>> @@ -1507,7 +1509,7 @@ static int iommu_alloc_default_domain(struct
>> iommu_group *group,
>>           if (group->default_domain)
>>                   return 0;
>>
>> -       type = iommu_get_def_domain_type(dev);
>> +       type = iommu_get_def_domain_type(dev) ? : iommu_def_domain_type;
>>
>>           return iommu_group_alloc_default_domain(dev->bus, group, type);
>>    }
>>
>> AFAICS the other two callers should also handle 0 correctly. Have you
>> seen a problem in practice?
> 
> Thanks for pointing out how the return value is being handled by the callers.
> However, the same check is missing in probe_get_default_domain_type():
> static int probe_get_default_domain_type(struct device *dev, void *data)
> {
>          struct __group_domain_type *gtype = data;
>          unsigned int type = iommu_get_def_domain_type(dev);
> ...
> }

I'm still not following - the next line right after that is "if (type)", 
which means it won't touch gtype, and if that happens for every 
iteration, probe_alloc_default_domain() subsequently hits its "if 
(!gtype.type)" condition and still ends up with iommu_def_domain_type. 
This *was* one of the other two callers I was talking about (the second 
being iommu_change_dev_def_domain()), and in fact on second look I think 
your proposed change will actually break this logic, since it's 
necessary to differentiate between a specific type being requested for 
the given device, and a "don't care" response which only implies to use 
the global default type if it's still standing after *all* the 
appropriate devices have been queried.

> I personally prefer the old way instead of open coding with ternary
> operator, so I'll do that in v2.
> 
> In practice, this causes a kernel panic when probing Realtek WiFi.
> Because of the bug, dma_ops isn't set by probe_finalize(),
> dma_map_single() falls back to swiotlb which isn't set and caused a
> kernel panic.

Hmm, but if that's the case, wouldn't it still be a problem anyway if 
the end result was IOMMU_DOMAIN_IDENTITY? I can't claim to fully 
understand the x86 swiotlb and no_iommu dance, but nothing really stands 
out to give me confidence that it handles the passthrough options correctly.

Robin.

> I didn't attach the panic log because the system simply is frozen at
> that point so the message is not logged to the storage.
> I'll see if I can find another way to collect the log and attach it in v2.
> 
> Kai-Heng
> 
>>
>> Robin.
>>
>>> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
>>> ---
>>>    drivers/iommu/iommu.c | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>> index 5419c4b9f27a..faac4f795025 100644
>>> --- a/drivers/iommu/iommu.c
>>> +++ b/drivers/iommu/iommu.c
>>> @@ -1507,14 +1507,15 @@ EXPORT_SYMBOL_GPL(fsl_mc_device_group);
>>>    static int iommu_get_def_domain_type(struct device *dev)
>>>    {
>>>        const struct iommu_ops *ops = dev->bus->iommu_ops;
>>> +     unsigned int type = 0;
>>>
>>>        if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
>>>                return IOMMU_DOMAIN_DMA;
>>>
>>>        if (ops->def_domain_type)
>>> -             return ops->def_domain_type(dev);
>>> +             type = ops->def_domain_type(dev);
>>>
>>> -     return 0;
>>> +     return (type == 0) ? iommu_def_domain_type : type;
>>>    }
>>>
>>>    static int iommu_group_alloc_default_domain(struct bus_type *bus,
>>>