[RFC PATCH 2/2] dma-direct: Fix dma_direct_{alloc,free}() for Hyperv-V IVMs

[Andrea Parri (Microsoft)]

In Hyper-V AMD SEV-SNP Isolated VMs, the virtual address returned by
dma_direct_alloc() must map above dma_unencrypted_base because the
memory is shared with the hardware device and must not be encrypted.

Modify dma_direct_alloc() to do the necessary remapping.  In
dma_direct_free(), use the (unmodified) DMA address to derive the
original virtual address and re-encrypt the pages.

Suggested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Co-developed-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
---
 kernel/dma/direct.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 06b2b901e37a3..c4ce277687a49 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -13,6 +13,7 @@
 #include <linux/vmalloc.h>
 #include <linux/set_memory.h>
 #include <linux/slab.h>
+#include <linux/io.h> /* for memremap() */
 #include "direct.h"
 
 /*
@@ -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
 	}
 
 	memset(ret, 0, size);
@@ -360,11 +376,23 @@ void dma_direct_free(struct device *dev, size_t size,
 	    dma_free_from_pool(dev, cpu_addr, PAGE_ALIGN(size)))
 		return;
 
-	if (is_vmalloc_addr(cpu_addr)) {
+	/*
+	 * 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))
 			return;
 	}
-- 
2.25.1