diff mbox series

[v2,4/4] arm: replace vector mem type with read-only type

Message ID 20201023091437.8225-5-miles.chen@mediatek.com (mailing list archive)
State New, archived
Headers show
Series arm: support get_user_pages_fast | expand

Commit Message

Miles Chen Oct. 23, 2020, 9:14 a.m. UTC
Since kernel no longer writes to the vector, try to replace
the vector mem type with read-only type and remove L_PTE_MT_VECTORS.

from Catalin in [1]:
"
> I don't think this matters since the kernel no longer writes to the
> vectors page at run-time but it needs cleaning up a bit (and testing in
> case I missed something). IOW, do we still need a dedicated mapping type
> for the vectors or we can simply use the read-only user page attributes?
"

[1] https://lore.kernel.org/patchwork/patch/986574/

Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
---
 arch/arm/include/asm/pgtable-2level.h | 1 -
 arch/arm/mm/mmu.c                     | 7 +++----
 arch/arm/mm/proc-macros.S             | 2 +-
 3 files changed, 4 insertions(+), 6 deletions(-)

Comments

Russell King - ARM Linux admin Oct. 23, 2020, 10:12 a.m. UTC | #1
On Fri, Oct 23, 2020 at 05:14:37PM +0800, Miles Chen wrote:
> Since kernel no longer writes to the vector, try to replace
> the vector mem type with read-only type and remove L_PTE_MT_VECTORS.
> 
> from Catalin in [1]:
> "
> > I don't think this matters since the kernel no longer writes to the
> > vectors page at run-time but it needs cleaning up a bit (and testing in
> > case I missed something). IOW, do we still need a dedicated mapping type
> > for the vectors or we can simply use the read-only user page attributes?
> "

Catalin is incorrect. If CONFIG_KUSER_HELPERS is enabled, then the
vectors page is definitely written to - it's a user interface, so
it's not going to change:

#ifdef CONFIG_KUSER_HELPERS
                        /*
                         * User space must never try to access this
                         * directly.  Expect your app to break
                         * eventually if you do so.  The user helper
                         * at 0xffff0fe0 must be used instead.  (see
                         * entry-armv.S for details)
                         */
                        *((unsigned int *)0xffff0ff0) = val;
#endif
Miles Chen Oct. 27, 2020, 7:41 a.m. UTC | #2
On Fri, 2020-10-23 at 11:12 +0100, Russell King - ARM Linux admin wrote:
> On Fri, Oct 23, 2020 at 05:14:37PM +0800, Miles Chen wrote:
> > Since kernel no longer writes to the vector, try to replace
> > the vector mem type with read-only type and remove L_PTE_MT_VECTORS.
> > 
> > from Catalin in [1]:
> > "
> > > I don't think this matters since the kernel no longer writes to the
> > > vectors page at run-time but it needs cleaning up a bit (and testing in
> > > case I missed something). IOW, do we still need a dedicated mapping type
> > > for the vectors or we can simply use the read-only user page attributes?
> > "
> 
> Catalin is incorrect. If CONFIG_KUSER_HELPERS is enabled, then the
> vectors page is definitely written to - it's a user interface, so
> it's not going to change:
> 
> #ifdef CONFIG_KUSER_HELPERS
>                         /*
>                          * User space must never try to access this
>                          * directly.  Expect your app to break
>                          * eventually if you do so.  The user helper
>                          * at 0xffff0fe0 must be used instead.  (see
>                          * entry-armv.S for details)
>                          */
>                         *((unsigned int *)0xffff0ff0) = val;
> #endif
> 

Thanks for the comment. 
We have to keep L_PTE_MT_VECTORS for KUSER_HELPERS.


Miles
diff mbox series

Patch

diff --git a/arch/arm/include/asm/pgtable-2level.h b/arch/arm/include/asm/pgtable-2level.h
index 385e7a32394e..438359d3675f 100644
--- a/arch/arm/include/asm/pgtable-2level.h
+++ b/arch/arm/include/asm/pgtable-2level.h
@@ -163,7 +163,6 @@ 
 #define L_PTE_MT_WRITETHROUGH	(_AT(pteval_t, 0x02) << 2)	/* 0010 */
 #define L_PTE_MT_WRITEBACK	(_AT(pteval_t, 0x03) << 2)	/* 0011 */
 #define L_PTE_MT_DEV_SHARED	(_AT(pteval_t, 0x04) << 2)	/* 0100 */
-#define L_PTE_MT_VECTORS	(_AT(pteval_t, 0x05) << 2)	/* 0101 */
 #define L_PTE_MT_MINICACHE	(_AT(pteval_t, 0x06) << 2)	/* 0110 (sa1100, xscale) */
 #define L_PTE_MT_WRITEALLOC	(_AT(pteval_t, 0x07) << 2)	/* 0111 */
 #if defined(CONFIG_CPU_V7) || defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_V6K)
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index ab69250a86bc..0b6b377e2cce 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -536,12 +536,11 @@  static void __init build_mem_type_table(void)
 
 #ifndef CONFIG_ARM_LPAE
 	/*
-	 * We don't use domains on ARMv6 (since this causes problems with
-	 * v6/v7 kernels), so we must use a separate memory type for user
-	 * r/o, kernel r/w to map the vectors page.
+	 * We no longer write to the vector pages ar run-time, to
+	 * use read-only user page attribute for vector.
 	 */
 	if (cpu_arch == CPU_ARCH_ARMv6)
-		vecs_pgprot |= L_PTE_MT_VECTORS;
+		vecs_pgprot |= L_PTE_RDONLY;
 
 	/*
 	 * Check is it with support for the PXN bit
diff --git a/arch/arm/mm/proc-macros.S b/arch/arm/mm/proc-macros.S
index dde1d6374250..f3e6551b4a7e 100644
--- a/arch/arm/mm/proc-macros.S
+++ b/arch/arm/mm/proc-macros.S
@@ -137,7 +137,7 @@ 
 	.long	PTE_CACHEABLE					@ L_PTE_MT_WRITETHROUGH
 	.long	PTE_CACHEABLE | PTE_BUFFERABLE			@ L_PTE_MT_WRITEBACK
 	.long	PTE_BUFFERABLE					@ L_PTE_MT_DEV_SHARED
-	.long	PTE_CACHEABLE | PTE_BUFFERABLE | PTE_EXT_APX	@ L_PTE_MT_VECTORS
+	.long	0x00						@ unused
 	.long	0x00						@ L_PTE_MT_MINICACHE (not present)
 	.long	PTE_EXT_TEX(1) | PTE_CACHEABLE | PTE_BUFFERABLE	@ L_PTE_MT_WRITEALLOC
 	.long	0x00						@ unused