linux-kernel - [RFC PATCH 6/8] arm64/Kconfig: Drop support for 48-bit virtual addressing

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Message-ID: <20241030101803.2037606-16-ardb+git@google.com>
Date: Wed, 30 Oct 2024 11:18:10 +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 6/8] arm64/Kconfig: Drop support for 48-bit virtual addressing

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

Drop support for 48-bit virtual addressing on 16k pages, which is the
only remaining config that supports this explicitly.

On 16k pages, 48-bit virtual addressing uses 4 levels of translation,
where the top level has only 2 entries. This is very inefficient in
terms of TLB utilization, and so 47-bit virtual addressing is usually a
better choice.

Note that x86 supports only 47-bit virtual addressing for user space
with 4 translation levels, due to the way its page tables are
constructed (a single combined root table for both kernel and user
space), and so removing this configuration is unlikely to create
portability concerns.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/Kconfig | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7df7d24c767d..39d0d2eb5b7c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -410,12 +410,12 @@ config BUILTIN_RETURN_ADDRESS_STRIPS_PAC
 config KASAN_SHADOW_OFFSET
 	hex
 	depends on KASAN_GENERIC || KASAN_SW_TAGS
-	default 0xdfff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && !KASAN_SW_TAGS
+	default 0xdfff800000000000 if ARM64_VA_BITS_52 && !ARM64_16K_PAGES && !KASAN_SW_TAGS
 	default 0xdfffc00000000000 if ARM64_VA_BITS_52 && ARM64_16K_PAGES && !KASAN_SW_TAGS
 	default 0xdffffe0000000000 if ARM64_VA_BITS_42 && !KASAN_SW_TAGS
 	default 0xdfffffc000000000 if ARM64_VA_BITS_39 && !KASAN_SW_TAGS
 	default 0xdffffff800000000 if ARM64_VA_BITS_36 && !KASAN_SW_TAGS
-	default 0xefff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && KASAN_SW_TAGS
+	default 0xefff800000000000 if ARM64_VA_BITS_52 && !ARM64_16K_PAGES && KASAN_SW_TAGS
 	default 0xefffc00000000000 if ARM64_VA_BITS_52 && ARM64_16K_PAGES && KASAN_SW_TAGS
 	default 0xeffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS
 	default 0xefffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS
@@ -1353,10 +1353,6 @@ config ARM64_VA_BITS_42
 	bool "42-bit"
 	depends on PAGE_SIZE_64KB
 
-config ARM64_VA_BITS_48
-	bool "48-bit"
-	depends on PAGE_SIZE_16KB
-
 config ARM64_VA_BITS_52
 	bool "52-bit"
 	help
@@ -1392,7 +1388,6 @@ config ARM64_VA_BITS
 	default 36 if ARM64_VA_BITS_36
 	default 39 if ARM64_VA_BITS_39
 	default 42 if ARM64_VA_BITS_42
-	default 48 if ARM64_VA_BITS_48
 	default 52 if ARM64_VA_BITS_52
 
 choice
-- 
2.47.0.163.g1226f6d8fa-goog