[RFC PATCH 1/2] swiotlb,dma-direct: Move swiotlb_unencrypted_base to direct.c

["Andrea Parri (Microsoft)" <parri.andrea@gmail.com>]

The variable will come in handy to enable dma_direct_{alloc,free}()
for Hyper-V AMD SEV-SNP Isolated VMs.

Rename swiotlb_unencrypted_base to dma_unencrypted_base to indicate
that the notion is not restricted to SWIOTLB.

No functional change.

Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
Yeah, this is in some sense trading the dependency on SWIOTLB for a
dependency on HAS_DMA:

Q1. I'm unable to envision a scenario where SWIOTLB without HAS_DMA
would make sense but I'm also expecting one of the kernel test bots
to try such a nonsensical configuration... should the references to
dma_unencrypted_base in swiotlb.c be protected with HAS_DMA? other?

Q2. Can the #ifdef CONFIG_HAS_DMA in arch/x86/kernel/cpu/mshyperv.c
be removed? can we make HYPERV "depends on HAS_DMA"?

...

 arch/x86/kernel/cpu/mshyperv.c |  6 +++---
 include/linux/dma-direct.h     |  2 ++
 include/linux/swiotlb.h        |  2 --
 kernel/dma/direct.c            |  8 ++++++++
 kernel/dma/swiotlb.c           | 12 +++++-------
 5 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 831613959a92a..47e9cece86ff8 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -18,7 +18,7 @@
 #include <linux/kexec.h>
 #include <linux/i8253.h>
 #include <linux/random.h>
-#include <linux/swiotlb.h>
+#include <linux/dma-direct.h>
 #include <asm/processor.h>
 #include <asm/hypervisor.h>
 #include <asm/hyperv-tlfs.h>
@@ -333,8 +333,8 @@ static void __init ms_hyperv_init_platform(void)
 
 		if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP) {
 			static_branch_enable(&isolation_type_snp);
-#ifdef CONFIG_SWIOTLB
-			swiotlb_unencrypted_base = ms_hyperv.shared_gpa_boundary;
+#ifdef CONFIG_HAS_DMA
+			dma_unencrypted_base = ms_hyperv.shared_gpa_boundary;
 #endif
 		}
 		/* Isolation VMs are unenlightened SEV-based VMs, thus this check: */
diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 18aade195884d..0b7e4c4b7b34c 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -14,6 +14,8 @@
 
 extern unsigned int zone_dma_bits;
 
+extern phys_addr_t dma_unencrypted_base;
+
 /*
  * Record the mapping of CPU physical to DMA addresses for a given region.
  */
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 7ed35dd3de6e7..fa2e85f21af61 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -190,6 +190,4 @@ static inline bool is_swiotlb_for_alloc(struct device *dev)
 }
 #endif /* CONFIG_DMA_RESTRICTED_POOL */
 
-extern phys_addr_t swiotlb_unencrypted_base;
-
 #endif /* __LINUX_SWIOTLB_H */
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 8d0b68a170422..06b2b901e37a3 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -22,6 +22,14 @@
  */
 unsigned int zone_dma_bits __ro_after_init = 24;
 
+/*
+ * Certain Confidential Computing solutions, such as Hyper-V AMD SEV-SNP
+ * isolated VMs, use dma_unencrypted_base as a watermark: memory addresses
+ * below dma_unencrypted_base are treated as private, while memory above
+ * dma_unencrypted_base is treated as shared.
+ */
+phys_addr_t dma_unencrypted_base;
+
 static inline dma_addr_t phys_to_dma_direct(struct device *dev,
 		phys_addr_t phys)
 {
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index cb50f8d383606..78d4f5294a56c 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -67,8 +67,6 @@ static bool swiotlb_force_disable;
 
 struct io_tlb_mem io_tlb_default_mem;
 
-phys_addr_t swiotlb_unencrypted_base;
-
 static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
 
 static int __init
@@ -142,7 +140,7 @@ static inline unsigned long nr_slots(u64 val)
 
 /*
  * Remap swioltb memory in the unencrypted physical address space
- * when swiotlb_unencrypted_base is set. (e.g. for Hyper-V AMD SEV-SNP
+ * when dma_unencrypted_base is set. (e.g. for Hyper-V AMD SEV-SNP
  * Isolation VMs).
  */
 #ifdef CONFIG_HAS_IOMEM
@@ -150,8 +148,8 @@ static void *swiotlb_mem_remap(struct io_tlb_mem *mem, unsigned long bytes)
 {
 	void *vaddr = NULL;
 
-	if (swiotlb_unencrypted_base) {
-		phys_addr_t paddr = mem->start + swiotlb_unencrypted_base;
+	if (dma_unencrypted_base) {
+		phys_addr_t paddr = mem->start + dma_unencrypted_base;
 
 		vaddr = memremap(paddr, bytes, MEMREMAP_WB);
 		if (!vaddr)
@@ -213,10 +211,10 @@ static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
 	}
 
 	/*
-	 * If swiotlb_unencrypted_base is set, the bounce buffer memory will
+	 * If dma_unencrypted_base is set, the bounce buffer memory will
 	 * be remapped and cleared in swiotlb_update_mem_attributes.
 	 */
-	if (swiotlb_unencrypted_base)
+	if (dma_unencrypted_base)
 		return;
 
 	memset(vaddr, 0, bytes);
-- 
2.25.1