Re: [PATCH v2] x86/mm: Expand static page table for fixmap space

[Juergen Gross <jgross@suse.com>]

On 17/09/18 10:39, Feng Tang wrote:
> We met a kernel panic when enabling earlycon, which is due to
> the fixmap address of earlycon is not statically setup.
> 
> Currently the static fixmap setup in head_64.S only covers 2M
> virtual address space, while it acutually could be in 4M space
> with different kernel configurations.
> 
> So increase the static space to 4M for now by defining FIXMAP_PMD_NUM
> to 2, and add a build time check for future possible overflow so that
> the macro could be increased accordingly.
> 
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>  arch/x86/include/asm/fixmap.h     |  3 +++
>  arch/x86/include/asm/pgtable_64.h |  3 ++-
>  arch/x86/kernel/head_64.S         | 16 ++++++++++++----
>  arch/x86/mm/pgtable.c             | 10 ++++++++++
>  arch/x86/xen/mmu_pv.c             |  7 +++++--
>  5 files changed, 32 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index e203169..ffaa0fa 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -14,6 +14,9 @@
>  #ifndef _ASM_X86_FIXMAP_H
>  #define _ASM_X86_FIXMAP_H
>  
> +/* Put here to be accessble for assembly code like head_64.S */
> +#define FIXMAP_PMD_NUM	2
> +
>  #ifndef __ASSEMBLY__
>  #include <linux/kernel.h>
>  #include <asm/acpi.h>
> diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
> index ce2b590..9c85b54 100644
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -14,6 +14,7 @@
>  #include <asm/processor.h>
>  #include <linux/bitops.h>
>  #include <linux/threads.h>
> +#include <asm/fixmap.h>
>  
>  extern p4d_t level4_kernel_pgt[512];
>  extern p4d_t level4_ident_pgt[512];
> @@ -22,7 +23,7 @@ extern pud_t level3_ident_pgt[512];
>  extern pmd_t level2_kernel_pgt[512];
>  extern pmd_t level2_fixmap_pgt[512];
>  extern pmd_t level2_ident_pgt[512];
> -extern pte_t level1_fixmap_pgt[512];
> +extern pte_t level1_fixmap_pgt[512 * FIXMAP_PMD_NUM];
>  extern pgd_t init_top_pgt[];
>  
>  #define swapper_pg_dir init_top_pgt
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 15ebc2f..a3618cf 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -24,6 +24,7 @@
>  #include "../entry/calling.h"
>  #include <asm/export.h>
>  #include <asm/nospec-branch.h>
> +#include <asm/fixmap.h>
>  
>  #ifdef CONFIG_PARAVIRT
>  #include <asm/asm-offsets.h>
> @@ -445,13 +446,20 @@ NEXT_PAGE(level2_kernel_pgt)
>  		KERNEL_IMAGE_SIZE/PMD_SIZE)
>  
>  NEXT_PAGE(level2_fixmap_pgt)
> -	.fill	506,8,0
> -	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
> -	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
> -	.fill	5,8,0
> +	.fill	(512 - 4 - FIXMAP_PMD_NUM),8,0
> +	pgtno = 0
> +	.rept (FIXMAP_PMD_NUM)
> +	.quad level1_fixmap_pgt + (pgtno << PAGE_SHIFT) - __START_KERNEL_map \
> +		+ _PAGE_TABLE_NOENC;
> +	pgtno = pgtno + 1
> +	.endr
> +	/* 6 MB reserved space + a 2MB hole */
> +	.fill	4,8,0
>  
>  NEXT_PAGE(level1_fixmap_pgt)
> +	.rept (FIXMAP_PMD_NUM)
>  	.fill	512,8,0
> +	.endr
>  
>  #undef PMDS
>  
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index ae39455..a4052b1 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -637,6 +637,16 @@ void __native_set_fixmap(enum fixed_addresses idx, pte_t pte)
>  {
>  	unsigned long address = __fix_to_virt(idx);
>  
> +#ifdef CONFIG_X86_64
> +       /*
> +	* In arch/x86/kernel/head_64.S, the static page table has
> +	* been setup for fixmap. Add a sanity compiling check
> +	* whether fixmap space has exceeded the capacity.
> +	*/
> +	BUILD_BUG_ON(__end_of_permanent_fixed_addresses
> +			> (FIXMAP_PMD_NUM * PTRS_PER_PMD));
> +#endif
> +
>  	if (idx >= __end_of_fixed_addresses) {
>  		BUG();
>  		return;
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 2fe5c9b..74c4e17 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -1907,7 +1907,7 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
>  	/* L3_k[511] -> level2_fixmap_pgt */
>  	convert_pfn_mfn(level3_kernel_pgt);
>  
> -	/* L3_k[511][506] -> level1_fixmap_pgt */
> +	/* L3_k[507, 508 - FIXMAP_PMD_NUM] -> level1_fixmap_pgt */

You messed up the comment. The [511] must not be dropped.
I would swap the "508 - FIXMAP_PMD_NUM" and "507" in order to start
with the lower index, and replace "," by "...":

+	/* L3_k[511][508-FIXMAP_PMD_NUM ... 507] -> level1_fixmap_pgt */

With that corrected you can have my:

Reviewed-by: Juergen Gross <jgross@suse.com> (Xen parts)


Juergen