[PATCH] arm64: mm: fix runtime fallback to 48-bt VA when 52-bit VA is enabled

[Mark Salter <msalter@redhat.com>]

I ran into an early boot soft lockup on a Qualcomm Amberwing using a v5.11
kernel configured for 52-bit VA. This turned into a panic with a v5.12-rc2
kernel.

The problem is that when we fall back to 48-bit VA, idmap_t0sz is not
updated. Later, the kvm hypervisor uses idmap_t0sz to set its tcr_el2 and
hangs (v5.11). After commit 1401bef703a4 ("arm64: mm: Always update TCR_EL1
from __cpu_set_tcr_t0sz()"), the kernel panics when trying to use the idmap
to call idmap_cpu_replace_ttbr1().

Oddly, other systems (thunderX2 and Ampere eMag) which don't support 52-bit
VA seem to handle the setting of an unsupported t0sz without any apparent
problems. Indeed, if one reads back the tcr written with t0sz==12, the
value read has t0sz==16. Not so with Amberwing.

Fixes: 90ec95cda91a ("arm64: mm: Introduce VA_BITS_MIN")
Signed-off-by: Mark Salter <msalter@redhat.com>
---
 arch/arm64/kernel/head.S | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 66b0e0b66e31..2bcbbb26292e 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -291,6 +291,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
 	 */
 	adrp	x0, idmap_pg_dir
 	adrp	x3, __idmap_text_start		// __pa(__idmap_text_start)
+	mov	x4, TCR_T0SZ(VA_BITS)
 
 #ifdef CONFIG_ARM64_VA_BITS_52
 	mrs_s	x6, SYS_ID_AA64MMFR2_EL1
@@ -299,6 +300,13 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
 	cbnz	x6, 1f
 #endif
 	mov	x5, #VA_BITS_MIN
+#ifdef CONFIG_ARM64_VA_BITS_52
+	mov	x4, TCR_T0SZ(VA_BITS_MIN)
+	adr_l	x6, idmap_t0sz
+	str	x4, [x6]
+	dmb	sy
+	dc	ivac, x6		// Invalidate potentially stale cache line
+#endif
 1:
 	adr_l	x6, vabits_actual
 	str	x5, [x6]
@@ -319,7 +327,7 @@ 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, x4			// default T0SZ small enough?
 	b.ge	1f			// .. then skip VA range extension
 
 	adr_l	x6, idmap_t0sz
-- 
2.27.0