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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Daniel Axtens <dja@axtens.net>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 2/3] powerpc: Define swapper_pg_dir[] in C
In-Reply-To: <871r8siyqm.fsf@dja-thinkpad.axtens.net>
References: <5838caffa269e0957c5a50cc85477876220298b0.1623063174.git.christophe.leroy@csgroup.eu>
 <5e3f1b8a4695c33ccc80aa3870e016bef32b85e1.1623063174.git.christophe.leroy@csgroup.eu>
 <871r8siyqm.fsf@dja-thinkpad.axtens.net>
Date:   Wed, 23 Jun 2021 22:38:58 +1000
Message-ID: <87czsc21st.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Daniel Axtens <dja@axtens.net> 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
>

...
>> 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