arm64: mm: Handle LVA support as a CPU feature

Currently, we detect CPU support for 52-bit virtual addressing (LVA)
extremely early, before creating the kernel page tables or enabling the
MMU. We cannot override the feature this early, and so large virtual
addressing is always enabled on CPUs that implement support for it if
the software support for it was enabled at build time. It also means we
rely on non-trivial code in asm to deal with this feature.

Given that both the ID map and the TTBR1 mapping of the kernel image are
guaranteed to be 48-bit addressable, it is not actually necessary to
enable support this early, and instead, we can model it as a CPU
feature. That way, we can rely on code patching to get the correct
TCR.T1SZ values programmed on secondary boot and resume from suspend.

On the primary boot path, we simply enable the MMU with 48-bit virtual
addressing initially, and update TCR.T1SZ if LVA is supported from C
code, right before creating the kernel mapping. Given that TTBR1 still
points to reserved_pg_dir at this point, updating TCR.T1SZ should be
safe without the need for explicit TLB maintenance.

Since this gets rid of all accesses to the vabits_actual variable from
asm code that occurred before TCR.T1SZ had been programmed, we no longer
have a need for this variable, and we can replace it with a C expression
that produces the correct value directly, based on the value of TCR.T1SZ.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-70-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 60904a6c4b42..9680d7444b3b 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -209,9 +209,20 @@
 #include <asm/boot.h>
 #include <asm/bug.h>
 #include <asm/sections.h>
+#include <asm/sysreg.h>
+
+static inline u64 __pure read_tcr(void)
+{
+	u64  tcr;
+
+	// read_sysreg() uses asm volatile, so avoid it here
+	asm("mrs %0, tcr_el1" : "=r"(tcr));
+	return tcr;
+}
 
 #if VA_BITS > 48
-extern u64			vabits_actual;
+// For reasons of #include hell, we can't use TCR_T1SZ_OFFSET/TCR_T1SZ_MASK here
+#define vabits_actual		(64 - ((read_tcr() >> 16) & 63))
 #else
 #define vabits_actual		((u64)VA_BITS)
 #endif