linux-kernel - Re: "Convert the AMD iommu driver to the dma-iommu api" is buggy

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Date:   Wed, 16 Oct 2019 17:26:24 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Qian Cai <cai@....pw>, Joerg Roedel <jroedel@...e.de>
Cc:     Tom Murphy <murphyt7@....ie>, Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: "Convert the AMD iommu driver to the dma-iommu api" is buggy

On 16/10/2019 17:11, Qian Cai wrote:
> On Wed, 2019-10-16 at 18:03 +0200, Joerg Roedel wrote:
>> On Wed, Oct 16, 2019 at 11:53:33AM -0400, Qian Cai wrote:
>>> On Wed, 2019-10-16 at 17:31 +0200, Joerg Roedel wrote:
>>> The x86 one might just be a mistake.
>>>
>>> diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
>>> index ad05484d0c80..63c4b894751d 100644
>>> --- a/drivers/iommu/amd_iommu.c
>>> +++ b/drivers/iommu/amd_iommu.c
>>> @@ -2542,7 +2542,7 @@ static int amd_iommu_map(struct iommu_domain *dom,
>>> unsigned long iova,
>>>          if (iommu_prot & IOMMU_WRITE)
>>>                  prot |= IOMMU_PROT_IW;
>>>   
>>> -       ret = iommu_map_page(domain, iova, paddr, page_size, prot, GFP_KERNEL);
>>> +       ret = iommu_map_page(domain, iova, paddr, page_size, prot, gfp);
>>
>> Yeah, that is a bug, I spotted that too.
>>
>>> @@ -1185,7 +1185,7 @@ static struct iommu_dma_msi_page
>>> *iommu_dma_get_msi_page(struct device *dev,
>>>          if (!iova)
>>>                  goto out_free_page;
>>>   
>>> -       if (iommu_map(domain, iova, msi_addr, size, prot))
>>> +       if (iommu_map_atomic(domain, iova, msi_addr, size, prot))
>>>                  goto out_free_iova;
>>
>> Not so sure this is a bug, this code is only about setting up MSIs on
>> ARM. It probably doesn't need to be atomic.
> 
> The patch "iommu: Add gfp parameter to iommu_ops::map" does this. It could be
> called from an atomic context as showed in the arm64 call traces,
> 
> +int iommu_map(struct iommu_domain *domain, unsigned long iova,
> +             phys_addr_t paddr, size_t size, int prot)
> +{
> +       might_sleep();
> +       return __iommu_map(domain, iova, paddr, size, prot, GFP_KERNEL);
> +}

Also note that it's *only* the might_sleep() at issue here - none of the 
arm64 IOMMU drivers have been converted to look at the new gfp argument 
yet, so anything they actually allocate while mapping will still be 
GFP_ATOMIC anyway.

(Carrying that flag down through the whole io-pgtable stack is on my 
to-do list...)

Robin.