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List:       linux-kernel
Subject:    Re: [RFC PATCH v4 19/28] swiotlb: Add warnings for use of bounce buffers with SME
From:       Tom Lendacky <thomas.lendacky () amd ! com>
Date:       2017-02-28 23:19:51
Message-ID: 9b5af67b-0969-5402-cc01-3ea98f41b748 () amd ! com
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On 2/27/2017 11:52 AM, Borislav Petkov wrote:
> On Thu, Feb 16, 2017 at 09:46:19AM -0600, Tom Lendacky wrote:
>> Add warnings to let the user know when bounce buffers are being used for
>> DMA when SME is active.  Since the bounce buffers are not in encrypted
>> memory, these notifications are to allow the user to determine some
>> appropriate action - if necessary.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/include/asm/mem_encrypt.h |   11 +++++++++++
>>  include/linux/dma-mapping.h        |   11 +++++++++++
>>  include/linux/mem_encrypt.h        |    6 ++++++
>>  lib/swiotlb.c                      |    3 +++
>>  4 files changed, 31 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
>> index 87e816f..5a17f1b 100644
>> --- a/arch/x86/include/asm/mem_encrypt.h
>> +++ b/arch/x86/include/asm/mem_encrypt.h
>> @@ -26,6 +26,11 @@ static inline bool sme_active(void)
>>  	return (sme_me_mask) ? true : false;
>>  }
>>
>> +static inline u64 sme_dma_mask(void)
>> +{
>> +	return ((u64)sme_me_mask << 1) - 1;
>> +}
>> +
>>  void __init sme_early_encrypt(resource_size_t paddr,
>>  			      unsigned long size);
>>  void __init sme_early_decrypt(resource_size_t paddr,
>> @@ -53,6 +58,12 @@ static inline bool sme_active(void)
>>  {
>>  	return false;
>>  }
>> +
>> +static inline u64 sme_dma_mask(void)
>> +{
>> +	return 0ULL;
>> +}
>> +
>>  #endif
>>
>>  static inline void __init sme_early_encrypt(resource_size_t paddr,
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index 10c5a17..130bef7 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -10,6 +10,7 @@
>>  #include <linux/scatterlist.h>
>>  #include <linux/kmemcheck.h>
>>  #include <linux/bug.h>
>> +#include <linux/mem_encrypt.h>
>>
>>  /**
>>   * List of possible attributes associated with a DMA mapping. The semantics
>> @@ -557,6 +558,11 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
>>
>>  	if (!dev->dma_mask || !dma_supported(dev, mask))
>>  		return -EIO;
>> +
>> +	if (sme_active() && (mask < sme_dma_mask()))
>> +		dev_warn(dev,
>> +			 "SME is active, device will require DMA bounce buffers\n");
>> +
>
> Yes, definitely _once() here.

Setting the mask is a probe/init type event, so I think not having the
_once() would be better so that all devices that set a mask to something
less than the SME encryption mask would be identified.  This isn't done
for every DMA, etc.

>
> It could be extended later to be per-device if the need arises.
>
> Also, a bit above in this function, we test if (ops->set_dma_mask) so
> device drivers which supply even an empty ->set_dma_mask will circumvent
> this check.
>
> It probably doesn't matter all that much right now because the
> only driver I see right now defining this method, though, is
> ethernet/intel/fm10k/fm10k_pf.c and some other arches' functionality
> which is unrelated here.

Device drivers don't supply set_dma_mask() since that is part of the
dma_map_ops structure. The fm10k_pf.c file function is unrelated to this
(it's part of an internal driver structure). The dma_map_ops structure
is setup by the arch or an iommu.

Thanks,
Tom

>
> But still...
>
>
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