linux-kernel - [PATCH RFC 2/2] ARM: nommu: remap exception base address to RAM

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Date:   Sun, 11 Dec 2016 18:42:55 +0530
From:   Afzal Mohammed <afzal.mohd.ma@...il.com>
To:     Russell King <linux@...linux.org.uk>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Afzal Mohammed <afzal.mohd.ma@...il.com>
Subject: [PATCH RFC 2/2] ARM: nommu: remap exception base address to RAM

Remap exception base address to start of RAM in Kernel in !MMU mode.

Based on existing Kconfig help, Kernel was expecting it to be
configured by external support. Also earlier it was not possible to
copy the exception table to start of RAM due to Kconfig dependency,
which has been fixed by a change prior to this.

Kernel text start at an offset of at least 32K to account for page
tables in MMU case. On a !MMU build too this space is kept aside, and
since 2 pages (8K) is the maximum for exception plus stubs, it can be
placed at the start of RAM.

Signed-off-by: Afzal Mohammed <afzal.mohd.ma@...il.com>
---

i am a bit shaky about this change, though it works here on Cortex-A9,
this probably would have to be made robust so as to not cause issue on
other v7-A's upon trying to do !MMU (this won't affect normal MMU boot),
or specifically where security extensions are not enabled. Also effect
of hypervisor extension also need to be considered. Please let know if
any better ways to handle this.

 arch/arm/Kconfig-nommu       | 6 +++---
 arch/arm/kernel/head-nommu.S | 6 ++++++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/arm/Kconfig-nommu b/arch/arm/Kconfig-nommu
index b7576349528c..f57fbe3d5eb0 100644
--- a/arch/arm/Kconfig-nommu
+++ b/arch/arm/Kconfig-nommu
@@ -46,9 +46,9 @@ config REMAP_VECTORS_TO_RAM
 	  If your CPU provides a remap facility which allows the exception
 	  vectors to be mapped to writable memory, say 'n' here.

-	  Otherwise, say 'y' here.  In this case, the kernel will require
-	  external support to redirect the hardware exception vectors to
-	  the writable versions located at DRAM_BASE.
+	  Otherwise, say 'y' here.  In this case, the kernel will
+	  redirect the hardware exception vectors to the writable
+	  versions located at DRAM_BASE.

 config ARM_MPU
        bool 'Use the ARM v7 PMSA Compliant MPU'
diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
index 6b4eb27b8758..ac31c9647830 100644
--- a/arch/arm/kernel/head-nommu.S
+++ b/arch/arm/kernel/head-nommu.S
@@ -158,6 +158,12 @@ __after_proc_init:
 	bic	r0, r0, #CR_V
 #endif
 	mcr	p15, 0, r0, c1, c0, 0		@ write control reg
+
+#ifdef CONFIG_REMAP_VECTORS_TO_RAM
+	mov	r3, #CONFIG_VECTORS_BASE	@ read VECTORS_BASE
+	mcr	p15, 0, r3, c12, c0, 0		@ write to VBAR
+#endif
+
 #elif defined (CONFIG_CPU_V7M)
 	/* For V7M systems we want to modify the CCR similarly to the SCTLR */
 #ifdef CONFIG_CPU_DCACHE_DISABLE
-- 
2.11.0