arm64: mm: increase VA range of identity map

The page size and the number of translation levels, and hence the supported
virtual address range, are build-time configurables on arm64 whose optimal
values are use case dependent. However, in the current implementation, if
the system's RAM is located at a very high offset, the virtual address range
needs to reflect that merely because the identity mapping, which is only used
to enable or disable the MMU, requires the extended virtual range to map the
physical memory at an equal virtual offset.

This patch relaxes that requirement, by increasing the number of translation
levels for the identity mapping only, and only when actually needed, i.e.,
when system RAM's offset is found to be out of reach at runtime.

Tested-by: Laura Abbott <lauraa@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 43 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index a9eee33dfa62..ecf2d060036b 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -64,6 +64,49 @@ static inline void cpu_set_reserved_ttbr0(void)
 	: "r" (ttbr));
 }
 
+/*
+ * TCR.T0SZ value to use when the ID map is active. Usually equals
+ * TCR_T0SZ(VA_BITS), unless system RAM is positioned very high in
+ * physical memory, in which case it will be smaller.
+ */
+extern u64 idmap_t0sz;
+
+static inline bool __cpu_uses_extended_idmap(void)
+{
+	return (!IS_ENABLED(CONFIG_ARM64_VA_BITS_48) &&
+		unlikely(idmap_t0sz != TCR_T0SZ(VA_BITS)));
+}
+
+static inline void __cpu_set_tcr_t0sz(u64 t0sz)
+{
+	unsigned long tcr;
+
+	if (__cpu_uses_extended_idmap())
+		asm volatile (
+		"	mrs	%0, tcr_el1	;"
+		"	bfi	%0, %1, %2, %3	;"
+		"	msr	tcr_el1, %0	;"
+		"	isb"
+		: "=&r" (tcr)
+		: "r"(t0sz), "I"(TCR_T0SZ_OFFSET), "I"(TCR_TxSZ_WIDTH));
+}
+
+/*
+ * Set TCR.T0SZ to the value appropriate for activating the identity map.
+ */
+static inline void cpu_set_idmap_tcr_t0sz(void)
+{
+	__cpu_set_tcr_t0sz(idmap_t0sz);
+}
+
+/*
+ * Set TCR.T0SZ to its default value (based on VA_BITS)
+ */
+static inline void cpu_set_default_tcr_t0sz(void)
+{
+	__cpu_set_tcr_t0sz(TCR_T0SZ(VA_BITS));
+}
+
 static inline void switch_new_context(struct mm_struct *mm)
 {
 	unsigned long flags;