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

Konrad Rzeszutek Wilk
2017-02-17T15:59:55Z

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-5C7GfCeVMHo@public.gmane.org>
> ---
> 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");

You can make it one line. But I am wondering if you should use
printk_ratelimit as this may fill the console up.

> +
> *dev->dma_mask = mask;
> return 0;
> }
> @@ -576,6 +582,11 @@ static inline int dma_set_coherent_mask(struct device *dev, u64 mask)
> {
> if (!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");

Ditto.
> +
> dev->coherent_dma_mask = mask;
> return 0;
> }
> diff --git a/include/linux/mem_encrypt.h b/include/linux/mem_encrypt.h
> index 14a7b9f..6829ff1 100644
> --- a/include/linux/mem_encrypt.h
> +++ b/include/linux/mem_encrypt.h
> @@ -28,6 +28,12 @@ static inline bool sme_active(void)
> {
> return false;
> }
> +
> +static inline u64 sme_dma_mask(void)
> +{
> + return 0ULL;
> +}
> +
> #endif
>
> #endif /* CONFIG_AMD_MEM_ENCRYPT */
> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> index c463067..aff9353 100644
> --- a/lib/swiotlb.c
> +++ b/lib/swiotlb.c
> @@ -509,6 +509,9 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
> if (no_iotlb_memory)
> panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
>
> + WARN_ONCE(sme_active(),
> + "SME is active and system is using DMA bounce buffers\n");

How does that help?

As in what can the user do with this?
> +
> mask = dma_get_seg_boundary(hwdev);
>
> tbl_dma_addr &= mask;
>