Re: [PATCH 1/5] arm64: mm: use a 48-bit ID map when possible on 52-bit VA builds

52-bit VA kernels can run on hardware that is only 48-bit capable, but
configure the ID map as 52-bit by default. This was not a problem until
recently, because the special T0SZ value for a 52-bit VA space was never
programmed into the TCR register anwyay, and because a 52-bit ID map
happens to use the same number of translation levels as a 48-bit one.

This behavior was changed by commit 1401bef703a4 ("arm64: mm: Always update
TCR_EL1 from __cpu_set_tcr_t0sz()"), which causes the unsupported T0SZ
value for a 52-bit VA to be programmed into TCR_EL1. While some hardware
simply ignores this, Mark reports that Amberwing systems choke on this,
resulting in a broken boot. But even before that commit, the unsupported
idmap_t0sz value was exposed to KVM and used to program TCR_EL2 incorrectly
as well.

Given that we already have to deal with address spaces being either 48-bit
or 52-bit in size, the cleanest approach seems to be to simply default to
a 48-bit VA ID map, and only switch to a 52-bit one if the placement of the
kernel in DRAM requires it. This is guaranteed not to happen unless the
system is actually 52-bit VA capable.

Fixes: 90ec95cda91a ("arm64: mm: Introduce VA_BITS_MIN")
Reported-by: Mark Salter <redacted>
Link: http://lore.kernel.org/r/20210310003216.410037-1-msalter@redhat.com (local)
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/mmu_context.h | 5 +----
 arch/arm64/kernel/head.S             | 2 +-
 arch/arm64/mm/mmu.c                  | 2 +-
 3 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 70ce8c1d2b07..0f467d550f27 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h

@@ -65,10 +65,7 @@ extern u64 idmap_ptrs_per_pgd;
 
 static inline bool __cpu_uses_extended_idmap(void)
 {
-	if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52))
-		return false;
-
-	return unlikely(idmap_t0sz != TCR_T0SZ(VA_BITS));
+	return unlikely(idmap_t0sz != TCR_T0SZ(vabits_actual));

 }
 
 /*

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 66b0e0b66e31..0b0387644dc0 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S

@@ -319,7 +319,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
 	 */
 	adrp	x5, __idmap_text_end
 	clz	x5, x5
-	cmp	x5, TCR_T0SZ(VA_BITS)	// default T0SZ small enough?
+	cmp	x5, TCR_T0SZ(VA_BITS_MIN) // default T0SZ small enough?
 	b.ge	1f			// .. then skip VA range extension