Re: [PATCH v6 25/34] swiotlb: Add warnings for use of bounce buffers with SME

[Borislav Petkov]

On Wed, Jun 07, 2017 at 02:17:32PM -0500, 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@xxxxxxx>
> ---
>  arch/x86/include/asm/mem_encrypt.h |    8 ++++++++
>  include/asm-generic/mem_encrypt.h  |    5 +++++
>  include/linux/dma-mapping.h        |    9 +++++++++
>  lib/swiotlb.c                      |    3 +++
>  4 files changed, 25 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> index f1215a4..c7a2525 100644
> --- a/arch/x86/include/asm/mem_encrypt.h
> +++ b/arch/x86/include/asm/mem_encrypt.h
> @@ -69,6 +69,14 @@ static inline bool sme_active(void)
>  	return !!sme_me_mask;
>  }
>  
> +static inline u64 sme_dma_mask(void)
> +{
> +	if (!sme_me_mask)
> +		return 0ULL;
> +
> +	return ((u64)sme_me_mask << 1) - 1;
> +}
> +
>  /*
>   * The __sme_pa() and __sme_pa_nodebug() macros are meant for use when
>   * writing to or comparing values from the cr3 register.  Having the
> diff --git a/include/asm-generic/mem_encrypt.h b/include/asm-generic/mem_encrypt.h
> index b55c3f9..fb02ff0 100644
> --- a/include/asm-generic/mem_encrypt.h
> +++ b/include/asm-generic/mem_encrypt.h
> @@ -22,6 +22,11 @@ static inline bool sme_active(void)
>  	return false;
>  }
>  
> +static inline u64 sme_dma_mask(void)
> +{
> +	return 0ULL;
> +}
> +
>  /*
>   * The __sme_set() and __sme_clr() macros are useful for adding or removing
>   * the encryption mask from a value (e.g. when dealing with pagetable
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 4f3eece..e2c5fda 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
> @@ -577,6 +578,10 @@ 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");

Something looks strange here:

you're checking sme_active() before calling sme_dma_mask() and yet in
it, you're checking !sme_me_mask again. What gives?

Why not move the sme_active() check into sme_dma_mask() and thus
simplify callers?

-- 
Regards/Gruss,
    Boris.

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