linux-kernel - [PATCH v3 1/2] x86/devmem: Remove duplicate range_is

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Message-ID: <174491712068.1395340.11679806523938235782.stgit@dwillia2-xfh.jf.intel.com>
Date: Thu, 17 Apr 2025 12:12:00 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: <dave.hansen@...ux.intel.com>
CC: Ingo Molnar <mingo@...nel.org>, Naveen N Rao <naveen@...nel.org>, "Nikolay
 Borisov" <nik.borisov@...e.com>, <linux-kernel@...r.kernel.org>,
	<linux-coco@...ts.linux.dev>
Subject: [PATCH v3 1/2] x86/devmem: Remove duplicate range_is_allowed()
 definition

17 years ago, Venki suggested [1] "A future improvement would be to
avoid the range_is_allowed duplication".

The only thing preventing a common implementation is that
phys_mem_access_prot_allowed() expects the range check to exit
immediately when PAT is disabled [2]. I.e. there is no cache conflict to
manage in that case. This cleanup was noticed on the path to
considering changing range_is_allowed() policy to blanket deny /dev/mem
for private (confidential computing) memory.

Note, however that phys_mem_access_prot_allowed() has long since stopped
being relevant for managing cache-type validation due to [3], and [4].

Commit 0124cecfc85a ("x86, PAT: disable /dev/mem mmap RAM with PAT") [1]
Commit 9e41bff2708e ("x86: fix /dev/mem mmap breakage when PAT is disabled") [2]
Commit 1886297ce0c8 ("x86/mm/pat: Fix BUG_ON() in mmap_mem() on QEMU/i386") [3]
Commit 0c3c8a18361a ("x86, PAT: Remove duplicate memtype reserve in devmem mmap") [4]

Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: "Naveen N Rao" <naveen@...nel.org>
Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 arch/x86/mm/pat/memtype.c |   31 ++++---------------------------
 drivers/char/mem.c        |   18 ------------------
 include/linux/io.h        |   21 +++++++++++++++++++++
 3 files changed, 25 insertions(+), 45 deletions(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 72d8cbc61158..c97b6598f187 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -38,6 +38,7 @@
 #include <linux/kernel.h>
 #include <linux/pfn_t.h>
 #include <linux/slab.h>
+#include <linux/io.h>
 #include <linux/mm.h>
 #include <linux/highmem.h>
 #include <linux/fs.h>
@@ -773,38 +774,14 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 	return vma_prot;
 }
 
-#ifdef CONFIG_STRICT_DEVMEM
-/* This check is done in drivers/char/mem.c in case of STRICT_DEVMEM */
-static inline int range_is_allowed(unsigned long pfn, unsigned long size)
-{
-	return 1;
-}
-#else
-/* This check is needed to avoid cache aliasing when PAT is enabled */
-static inline int range_is_allowed(unsigned long pfn, unsigned long size)
-{
-	u64 from = ((u64)pfn) << PAGE_SHIFT;
-	u64 to = from + size;
-	u64 cursor = from;
-
-	if (!pat_enabled())
-		return 1;
-
-	while (cursor < to) {
-		if (!devmem_is_allowed(pfn))
-			return 0;
-		cursor += PAGE_SIZE;
-		pfn++;
-	}
-	return 1;
-}
-#endif /* CONFIG_STRICT_DEVMEM */
-
 int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
 				unsigned long size, pgprot_t *vma_prot)
 {
 	enum page_cache_mode pcm = _PAGE_CACHE_MODE_WB;
 
+	if (!pat_enabled())
+		return 1;
+
 	if (!range_is_allowed(pfn, size))
 		return 0;
 
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 169eed162a7f..48839958b0b1 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -61,29 +61,11 @@ static inline int page_is_allowed(unsigned long pfn)
 {
 	return devmem_is_allowed(pfn);
 }
-static inline int range_is_allowed(unsigned long pfn, unsigned long size)
-{
-	u64 from = ((u64)pfn) << PAGE_SHIFT;
-	u64 to = from + size;
-	u64 cursor = from;
-
-	while (cursor < to) {
-		if (!devmem_is_allowed(pfn))
-			return 0;
-		cursor += PAGE_SIZE;
-		pfn++;
-	}
-	return 1;
-}
 #else
 static inline int page_is_allowed(unsigned long pfn)
 {
 	return 1;
 }
-static inline int range_is_allowed(unsigned long pfn, unsigned long size)
-{
-	return 1;
-}
 #endif
 
 static inline bool should_stop_iteration(void)
diff --git a/include/linux/io.h b/include/linux/io.h
index 6a6bc4d46d0a..0642c7ee41db 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -183,4 +183,25 @@ static inline void arch_io_free_memtype_wc(resource_size_t base,
 int devm_arch_io_reserve_memtype_wc(struct device *dev, resource_size_t start,
 				    resource_size_t size);
 
+#ifdef CONFIG_STRICT_DEVMEM
+static inline int range_is_allowed(unsigned long pfn, unsigned long size)
+{
+	u64 from = ((u64)pfn) << PAGE_SHIFT;
+	u64 to = from + size;
+	u64 cursor = from;
+
+	while (cursor < to) {
+		if (!devmem_is_allowed(pfn))
+			return 0;
+		cursor += PAGE_SIZE;
+		pfn++;
+	}
+	return 1;
+}
+#else
+static inline int range_is_allowed(unsigned long pfn, unsigned long size)
+{
+	return 1;
+}
+#endif
 #endif /* _LINUX_IO_H */