arm64: mm: Add confidential computing hook to ioremap_prot()

Confidential Computing environments such as pKVM and Arm's CCA
distinguish between shared (i.e. emulated) and private (i.e. assigned)
MMIO regions.

Introduce a hook into our implementation of ioremap_prot() so that MMIO
regions can be shared if necessary.

Reviewed-by: Catalin Marinas <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Acked-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: willdeacon

arch/arm64/include/asm/io.h

@@ -271,6 +271,10 @@ __iowrite64_copy(void __iomem *to, const void *from, size_t count)
  * I/O memory mapping functions.
  */
 
+typedef int (*ioremap_prot_hook_t)(phys_addr_t phys_addr, size_t size,
+				   pgprot_t *prot);
+int arm64_ioremap_prot_hook_register(const ioremap_prot_hook_t hook);
+
 #define ioremap_prot ioremap_prot
 
 #define _PAGE_IOREMAP PROT_DEVICE_nGnRE

arch/arm64/mm/ioremap.c

@@ -3,10 +3,22 @@
 #include <linux/mm.h>
 #include <linux/io.h>
 
+static ioremap_prot_hook_t ioremap_prot_hook;
+
+int arm64_ioremap_prot_hook_register(ioremap_prot_hook_t hook)
+{
+	if (WARN_ON(ioremap_prot_hook))
+		return -EBUSY;
+
+	ioremap_prot_hook = hook;
+	return 0;
+}
+
 void __iomem *ioremap_prot(phys_addr_t phys_addr, size_t size,
 			   unsigned long prot)
 {
 	unsigned long last_addr = phys_addr + size - 1;
+	pgprot_t pgprot = __pgprot(prot);
 
 	/* Don't allow outside PHYS_MASK */
 	if (last_addr & ~PHYS_MASK)
@@ -16,7 +28,16 @@ void __iomem *ioremap_prot(phys_addr_t phys_addr, size_t size,
 	if (WARN_ON(pfn_is_map_memory(__phys_to_pfn(phys_addr))))
 		return NULL;
 
-	return generic_ioremap_prot(phys_addr, size, __pgprot(prot));
+	/*
+	 * If a hook is registered (e.g. for confidential computing
+	 * purposes), call that now and barf if it fails.
+	 */
+	if (unlikely(ioremap_prot_hook) &&
+	    WARN_ON(ioremap_prot_hook(phys_addr, size, &pgprot))) {
+		return NULL;
+	}
+
+	return generic_ioremap_prot(phys_addr, size, pgprot);
 }
 EXPORT_SYMBOL(ioremap_prot);