linux-kernel - [RFC PATCH 3/8] arm64: Kconfig: eliminate 64k/48-bit VA combination

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Message-ID: <20241030101803.2037606-13-ardb+git@google.com>
Date: Wed, 30 Oct 2024 11:18:07 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Ryan Roberts <ryan.roberts@....com>, 
	Anshuman Khandual <anshuman.khandual@....com>, Kees Cook <keescook@...omium.org>
Subject: [RFC PATCH 3/8] arm64: Kconfig: eliminate 64k/48-bit VA combination

From: Ard Biesheuvel <ardb@...nel.org>

Now that the vmemmap region is sized dynamically based on the actual
size of the kernel VA space, there are no longer any material
differences between supporting 48-bit and 52-bit VA space sizes for 64k
pages, which use the same number of translation levels. And if needed,
52-bit virtual addressing can be disabled at boot on systems that do
support it but where 48-bit virtual addressing is preferred.

The only remaining difference is the size of a root level user page
table, which grows from 512 bytes to 8k when 52-bit virtual addressing
is enabled, but given that both are less than the size of a page, this
is easily fixed in the pgd_alloc init code. (In all other possible cases
where vabits_actual < VABITS holds, the effective PGD_SIZE equals the
page size, and so this change has no effect.)

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/Kconfig  | 5 +++--
 arch/arm64/mm/pgd.c | 9 +++++----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index ac8e7550430b..6a73fd61b4aa 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -379,11 +379,11 @@ config PGTABLE_LEVELS
 	int
 	default 2 if ARM64_16K_PAGES && ARM64_VA_BITS_36
 	default 2 if ARM64_64K_PAGES && ARM64_VA_BITS_42
-	default 3 if ARM64_64K_PAGES && (ARM64_VA_BITS_48 || ARM64_VA_BITS_52)
 	default 3 if ARM64_4K_PAGES && ARM64_VA_BITS_39
 	default 3 if ARM64_16K_PAGES && ARM64_VA_BITS_47
+	default 3 if ARM64_64K_PAGES
 	default 4 if ARM64_16K_PAGES && (ARM64_VA_BITS_48 || ARM64_VA_BITS_52)
-	default 4 if !ARM64_64K_PAGES && ARM64_VA_BITS_48
+	default 4 if ARM64_VA_BITS_48
 	default 5 if ARM64_4K_PAGES && ARM64_VA_BITS_52
 
 config ARCH_SUPPORTS_UPROBES
@@ -1361,6 +1361,7 @@ config ARM64_VA_BITS_47
 
 config ARM64_VA_BITS_48
 	bool "48-bit"
+	depends on !PAGE_SIZE_64KB
 
 config ARM64_VA_BITS_52
 	bool "52-bit"
diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
index 0c501cabc238..ecc4b1ec235c 100644
--- a/arch/arm64/mm/pgd.c
+++ b/arch/arm64/mm/pgd.c
@@ -48,20 +48,21 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
 
 void __init pgtable_cache_init(void)
 {
+	unsigned int size = PGD_SIZE >> (VA_BITS - vabits_actual);
+
 	if (pgdir_is_page_size())
 		return;
 
-#ifdef CONFIG_ARM64_PA_BITS_52
 	/*
 	 * With 52-bit physical addresses, the architecture requires the
 	 * top-level table to be aligned to at least 64 bytes.
 	 */
-	BUILD_BUG_ON(PGD_SIZE < 64);
-#endif
+	if (IS_ENABLED(CONFIG_ARM64_PA_BITS_52))
+		size = max(size, 64);
 
 	/*
 	 * Naturally aligned pgds required by the architecture.
 	 */
-	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_SIZE,
+	pgd_cache = kmem_cache_create("pgd_cache", size, size,
 				      SLAB_PANIC, NULL);
 }
-- 
2.47.0.163.g1226f6d8fa-goog