Daniel Axtens [off-list ref] writes:

Hi Christophe,

This breaks booting a radix KVM guest with 4k pages for me:

make pseries_le_defconfig
scripts/config -d CONFIG_PPC_64K_PAGES
scripts/config -e CONFIG_PPC_4K_PAGES
make vmlinux
sudo qemu-system-ppc64 -enable-kvm -M pseries -m 1G -nographic -vga none -smp 4 -cpu host -kernel vmlinux

Boot hangs after printing 'Booting Linux via __start()' and qemu's 'info
registers' reports that it's stuck at the instruction fetch exception.

My host is Power9, 64k page size radix, and
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34

...

quoted

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 730838c7ca39..79f2d1e61abd 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S

@@ -997,18 +997,3 @@ start_here_common:
 0:	trap
 	EMIT_BUG_ENTRY 0b, __FILE__, __LINE__, 0
 	.previous
-
-/*
- * We put a few things here that have to be page-aligned.
- * This stuff goes at the beginning of the bss, which is page-aligned.
- */
-	.section ".bss"
-/*
- * pgd dir should be aligned to PGD_TABLE_SIZE which is 64K.
- * We will need to find a better way to fix this
- */
-	.align	16
-
-	.globl	swapper_pg_dir
-swapper_pg_dir:
-	.space	PGD_TABLE_SIZE

This is now 4K aligned whereas it used to be 64K.

This fixes it and is not completely ugly?

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 1707ab580ee2..298469beaa90 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c

@@ -28,7 +28,13 @@
 #include <asm/hugetlb.h>
 #include <asm/pte-walk.h>
 
-pgd_t swapper_pg_dir[MAX_PTRS_PER_PGD] __page_aligned_bss;
+#ifdef CONFIG_PPC64
+#define PGD_ALIGN 0x10000
+#else
+#define PGD_ALIGN PAGE_SIZE
+#endif
+
+pgd_t swapper_pg_dir[MAX_PTRS_PER_PGD] __section(".bss..page_aligned") __aligned(PGD_ALIGN);
 
 static inline int is_exec_fault(void)
 {

cheers