Re: [RFC PATCH v2 10/32] x86: DMA support for SEV memory encryption

[Borislav Petkov]

On Thu, Mar 02, 2017 at 10:14:25AM -0500, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> DMA access to memory mapped as encrypted while SEV is active can not be
> encrypted during device write or decrypted during device read. In order
> for DMA to properly work when SEV is active, the swiotlb bounce buffers
> must be used.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/mm/mem_encrypt.c |   77 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 77 insertions(+)
> 
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 090419b..7df5f4c 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -197,8 +197,81 @@ void __init sme_early_init(void)
>  	/* Update the protection map with memory encryption mask */
>  	for (i = 0; i < ARRAY_SIZE(protection_map); i++)
>  		protection_map[i] = pgprot_encrypted(protection_map[i]);
> +
> +	if (sev_active())
> +		swiotlb_force = SWIOTLB_FORCE;
> +}
> +
> +static void *sme_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> +		       gfp_t gfp, unsigned long attrs)
> +{
> +	unsigned long dma_mask;
> +	unsigned int order;
> +	struct page *page;
> +	void *vaddr = NULL;
> +
> +	dma_mask = dma_alloc_coherent_mask(dev, gfp);
> +	order = get_order(size);
> +
> +	gfp &= ~__GFP_ZERO;

Please add a comment around here that swiotlb_alloc_coherent() will
memset(, 0, ) the memory. It took me a while to figure out what the
situation is.

Also, Joerg says the __GFP_ZERO is not absolutely necessary but it has
not been fixed in the other DMA alloc* functions because of fears that
something would break. That bit could also be part of the comment.

> +
> +	page = alloc_pages_node(dev_to_node(dev), gfp, order);
> +	if (page) {
> +		dma_addr_t addr;
> +
> +		/*
> +		 * Since we will be clearing the encryption bit, check the
> +		 * mask with it already cleared.
> +		 */
> +		addr = phys_to_dma(dev, page_to_phys(page)) & ~sme_me_mask;
> +		if ((addr + size) > dma_mask) {
> +			__free_pages(page, get_order(size));
> +		} else {
> +			vaddr = page_address(page);
> +			*dma_handle = addr;
> +		}
> +	}
> +
> +	if (!vaddr)
> +		vaddr = swiotlb_alloc_coherent(dev, size, dma_handle, gfp);
> +
> +	if (!vaddr)
> +		return NULL;
> +
> +	/* Clear the SME encryption bit for DMA use if not swiotlb area */
> +	if (!is_swiotlb_buffer(dma_to_phys(dev, *dma_handle))) {
> +		set_memory_decrypted((unsigned long)vaddr, 1 << order);
> +		*dma_handle &= ~sme_me_mask;
> +	}
> +
> +	return vaddr;
>  }
>  
> +static void sme_free(struct device *dev, size_t size, void *vaddr,
> +		     dma_addr_t dma_handle, unsigned long attrs)
> +{
> +	/* Set the SME encryption bit for re-use if not swiotlb area */
> +	if (!is_swiotlb_buffer(dma_to_phys(dev, dma_handle)))
> +		set_memory_encrypted((unsigned long)vaddr,
> +				     1 << get_order(size));
> +
> +	swiotlb_free_coherent(dev, size, vaddr, dma_handle);
> +}
> +
> +static struct dma_map_ops sme_dma_ops = {

WARNING: struct dma_map_ops should normally be const
#112: FILE: arch/x86/mm/mem_encrypt.c:261:
+static struct dma_map_ops sme_dma_ops = {

Please integrate scripts/checkpatch.pl in your patch creation workflow.
Some of the warnings/errors *actually* make sense.


> +	.alloc                  = sme_alloc,
> +	.free                   = sme_free,
> +	.map_page               = swiotlb_map_page,
> +	.unmap_page             = swiotlb_unmap_page,
> +	.map_sg                 = swiotlb_map_sg_attrs,
> +	.unmap_sg               = swiotlb_unmap_sg_attrs,
> +	.sync_single_for_cpu    = swiotlb_sync_single_for_cpu,
> +	.sync_single_for_device = swiotlb_sync_single_for_device,
> +	.sync_sg_for_cpu        = swiotlb_sync_sg_for_cpu,
> +	.sync_sg_for_device     = swiotlb_sync_sg_for_device,
> +	.mapping_error          = swiotlb_dma_mapping_error,
> +};
> +
>  /* Architecture __weak replacement functions */
>  void __init mem_encrypt_init(void)
>  {
> @@ -208,6 +281,10 @@ void __init mem_encrypt_init(void)
>  	/* Call into SWIOTLB to update the SWIOTLB DMA buffers */
>  	swiotlb_update_mem_attributes();
>  
> +	/* Use SEV DMA operations if SEV is active */

That's obvious. The WHY is not.

> +	if (sev_active())
> +		dma_ops = &sme_dma_ops;
> +
>  	pr_info("AMD Secure Memory Encryption (SME) active\n");
>  }
>  
> 

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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