Re: [RFC PATCH v3 13/20] x86: DMA support for memory encryption

[Radim KrÄmÃÅ]

2016-11-09 18:37-0600, Tom Lendacky:
> Since DMA addresses will effectively look like 48-bit addresses when the
> memory encryption mask is set, SWIOTLB is needed if the DMA mask of the
> device performing the DMA does not support 48-bits. SWIOTLB will be
> initialized to create un-encrypted bounce buffers for use by these devices.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
> diff --git a/arch/x86/kernel/pci-nommu.c b/arch/x86/kernel/pci-nommu.c
> @@ -30,7 +30,7 @@ static dma_addr_t nommu_map_page(struct device *dev, struct page *page,
>  				 enum dma_data_direction dir,
>  				 unsigned long attrs)
>  {
> -	dma_addr_t bus = page_to_phys(page) + offset;
> +	dma_addr_t bus = phys_to_dma(dev, page_to_phys(page)) + offset;
>  	WARN_ON(size == 0);
>  	if (!check_addr("map_single", dev, bus, size))
>  		return DMA_ERROR_CODE;
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> @@ -12,6 +12,8 @@
>  int swiotlb __read_mostly;
>  
>  void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> @@ -64,13 +66,15 @@ static struct dma_map_ops swiotlb_dma_ops = {
>   * pci_swiotlb_detect_override - set swiotlb to 1 if necessary
>   *
>   * This returns non-zero if we are forced to use swiotlb (by the boot
> - * option).
> + * option). If memory encryption is enabled then swiotlb will be set
> + * to 1 so that bounce buffers are allocated and used for devices that
> + * do not support the addressing range required for the encryption mask.
>   */
>  int __init pci_swiotlb_detect_override(void)
>  {
>  	int use_swiotlb = swiotlb | swiotlb_force;
>  
> -	if (swiotlb_force)
> +	if (swiotlb_force || sme_me_mask)
>  		swiotlb = 1;
>  
>  	return use_swiotlb;

We want to return 1 even if only sme_me_mask is 1, because the return
value is used for detection.  The following would be less obscure, IMO:

	if (swiotlb_force || sme_me_mask)
		swiotlb = 1;

	return swiotlb;

> diff --git a/init/main.c b/init/main.c
> @@ -598,6 +602,15 @@ asmlinkage __visible void __init start_kernel(void)
>  	 */
>  	locking_selftest();
>  
> +	/*
> +	 * This needs to be called before any devices perform DMA
> +	 * operations that might use the swiotlb bounce buffers.
> +	 * This call will mark the bounce buffers as un-encrypted so
> +	 * that their usage will not cause "plain-text" data to be
> +	 * decrypted when accessed.
> +	 */
> +	mem_encrypt_init();

(Comments below are connected to the reason why we call this.)

> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> @@ -159,6 +171,31 @@ void swiotlb_print_info(void)
> +/*
> + * If memory encryption is active, the DMA address for an encrypted page may
> + * be beyond the range of the device. If bounce buffers are required be sure
> + * that they are not on an encrypted page. This should be called before the
> + * iotlb area is used.
> + */
> +void __init swiotlb_clear_encryption(void)
> +{
> +	void *vaddr;
> +	unsigned long bytes;
> +
> +	if (no_iotlb_memory || !io_tlb_start || late_alloc)

io_tlb_start seems redundant -- when can !no_iotlb_memory &&
!io_tlb_start happen?

Is the order of calls
  1) swiotlb init
  2) SME init
  3) swiotlb late init 
?

We setup encrypted swiotlb and then decrypt it, but sometimes set it up
decrypted (late_alloc) ... why isn't the swiotlb set up decrypted
directly?

> +		return;
> +
> +	vaddr = phys_to_virt(io_tlb_start);
> +	bytes = PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT);
> +	swiotlb_set_mem_unenc(vaddr, bytes);
> +	memset(vaddr, 0, bytes);
> +
> +	vaddr = phys_to_virt(io_tlb_overflow_buffer);
> +	bytes = PAGE_ALIGN(io_tlb_overflow);
> +	swiotlb_set_mem_unenc(vaddr, bytes);
> +	memset(vaddr, 0, bytes);
> +}
> +
> @@ -541,7 +583,7 @@ static phys_addr_t
>  map_single(struct device *hwdev, phys_addr_t phys, size_t size,
>  	   enum dma_data_direction dir)
>  {
> -	dma_addr_t start_dma_addr = phys_to_dma(hwdev, io_tlb_start);
> +	dma_addr_t start_dma_addr = swiotlb_phys_to_dma(hwdev, io_tlb_start);

We have decrypted io_tlb_start before, so shouldn't its physical address
be saved without the sme bit?  (Which changes a lot ...)

Thanks.