Re: [RFC PATCH v4 19/28] swiotlb: Add warnings for use of bounce buffers with SME

Borislav Petkov
2017-02-27T17:52:59Z

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.

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.

But still...

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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