quoted

@@ -305,6 +306,21 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 		ret = page_address(page);
 		if (dma_set_decrypted(dev, ret, size))
 			goto out_free_pages;
+#ifdef CONFIG_HAS_IOMEM
+		/*
+		 * Remap the pages in the unencrypted physical address space
+		 * when dma_unencrypted_base is set (e.g., for Hyper-V AMD
+		 * SEV-SNP isolated guests).
+		 */
+		if (dma_unencrypted_base) {
+			phys_addr_t ret_pa = virt_to_phys(ret);
+
+			ret_pa += dma_unencrypted_base;
+			ret = memremap(ret_pa, size, MEMREMAP_WB);
+			if (!ret)
+				goto out_encrypt_pages;
+		}
+#endif

So:

this needs to move into dma_set_decrypted, otherwise we don't handle
the dma_alloc_pages case (never mind that this is pretty unreadable).

Which then again largely duplicates the code in swiotlb.  So I think
what we need here is a low-level helper that does the
set_memory_decrypted and memremap.  I'm not quite sure where it
should go, but maybe some of the people involved with memory
encryption might have good ideas.  unencrypted_base should go with
it and then both swiotlb and dma-direct can call it.

Agreed, will look into this more  (other people's ideas welcome).

quoted

+	/*
+	 * If dma_unencrypted_base is set, the virtual address returned by
+	 * dma_direct_alloc() is in the vmalloc address range.
+	 */
+	if (!dma_unencrypted_base && is_vmalloc_addr(cpu_addr)) {
 		vunmap(cpu_addr);
 	} else {
 		if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED))
 			arch_dma_clear_uncached(cpu_addr, size);
+#ifdef CONFIG_HAS_IOMEM
+		if (dma_unencrypted_base) {
+			memunmap(cpu_addr);
+			/* re-encrypt the pages using the original address */
+			cpu_addr = page_address(pfn_to_page(PHYS_PFN(
+					dma_to_phys(dev, dma_addr))));
+		}
+#endif
 		if (dma_set_encrypted(dev, cpu_addr, size))

Same on the unmap side.  It might also be worth looking into reordering
the checks in some form instead o that raw dma_unencrypted_base check
before the unmap.

Got it.

Thanks,
  Andrea