[PATCH] x86/xen: Build identity mapping page tables dynamically for XENPV

[PATCH] x86/xen: Build identity mapping page tables dynamically for XENPV

[Hou Wenlong]

After commit 47ffe0578aee ("x86/pvh: Add 64bit relocation page tables"),
the PVH entry uses a new set of page tables instead of the
preconstructed page tables in head64.S. Since those preconstructed page
tables are only used in XENPV now and XENPV does not actually need the
preconstructed identity page tables directly, they can be filled in
xen_setup_kernel_pagetable(). Therefore, build the identity mapping page
table dynamically to remove the preconstructed page tables and make the
code cleaner.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
---
 arch/x86/include/asm/pgtable_64.h |  2 --
 arch/x86/kernel/head_64.S         | 28 ----------------------------
 arch/x86/xen/mmu_pv.c             |  9 +++++++++
 3 files changed, 9 insertions(+), 30 deletions(-)

diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index f06e5d6a2747..ce45882ccd07 100644
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -19,10 +19,8 @@
 extern p4d_t level4_kernel_pgt[512];
 extern p4d_t level4_ident_pgt[512];
 extern pud_t level3_kernel_pgt[512];
-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 * FIXMAP_PMD_NUM];
 extern pgd_t init_top_pgt[];
 
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 21816b48537c..85d4a5094f6b 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -616,38 +616,10 @@ SYM_DATA(early_recursion_flag, .long 0)
 
 	.data
 
-#if defined(CONFIG_XEN_PV) || defined(CONFIG_PVH)
-SYM_DATA_START_PTI_ALIGNED(init_top_pgt)
-	.quad   level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
-	.org    init_top_pgt + L4_PAGE_OFFSET*8, 0
-	.quad   level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
-	.org    init_top_pgt + L4_START_KERNEL*8, 0
-	/* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
-	.quad   level3_kernel_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
-	.fill	PTI_USER_PGD_FILL,8,0
-SYM_DATA_END(init_top_pgt)
-
-SYM_DATA_START_PAGE_ALIGNED(level3_ident_pgt)
-	.quad	level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
-	.fill	511, 8, 0
-SYM_DATA_END(level3_ident_pgt)
-SYM_DATA_START_PAGE_ALIGNED(level2_ident_pgt)
-	/*
-	 * Since I easily can, map the first 1G.
-	 * Don't set NX because code runs from these pages.
-	 *
-	 * Note: This sets _PAGE_GLOBAL despite whether
-	 * the CPU supports it or it is enabled.  But,
-	 * the CPU should ignore the bit.
-	 */
-	PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
-SYM_DATA_END(level2_ident_pgt)
-#else
 SYM_DATA_START_PTI_ALIGNED(init_top_pgt)
 	.fill	512,8,0
 	.fill	PTI_USER_PGD_FILL,8,0
 SYM_DATA_END(init_top_pgt)
-#endif
 
 SYM_DATA_START_PAGE_ALIGNED(level4_kernel_pgt)
 	.fill	511,8,0
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 9fa00c4a8858..7d77c233002b 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -105,6 +105,9 @@ pte_t xen_make_pte_init(pteval_t pte);
 static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
 #endif
 
+static pud_t level3_ident_pgt[PTRS_PER_PUD] __page_aligned_bss;
+static pmd_t level2_ident_pgt[PTRS_PER_PMD] __page_aligned_bss;
+
 /*
  * Protects atomic reservation decrease/increase against concurrent increases.
  * Also protects non-atomic updates of current_pages and balloon lists.
@@ -1773,6 +1776,12 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
 	/* Zap identity mapping */
 	init_top_pgt[0] = __pgd(0);
 
+	init_top_pgt[pgd_index(__PAGE_OFFSET_BASE_L4)].pgd =
+		__pa_symbol(level3_ident_pgt) + _KERNPG_TABLE_NOENC;
+	init_top_pgt[pgd_index(__START_KERNEL_map)].pgd =
+		__pa_symbol(level3_kernel_pgt) + _PAGE_TABLE_NOENC;
+	level3_ident_pgt[0].pud = __pa_symbol(level2_ident_pgt) + _KERNPG_TABLE_NOENC;
+
 	/* Pre-constructed entries are in pfn, so convert to mfn */
 	/* L4[273] -> level3_ident_pgt  */
 	/* L4[511] -> level3_kernel_pgt */
-- 
2.31.1

Re: [PATCH] x86/xen: Build identity mapping page tables dynamically for XENPV

[Borislav Petkov]

On Thu, Jan 22, 2026 at 06:06:14PM +0800, Hou Wenlong wrote:
> After commit 47ffe0578aee ("x86/pvh: Add 64bit relocation page tables"),
> the PVH entry uses a new set of page tables instead of the
> preconstructed page tables in head64.S. Since those preconstructed page
> tables are only used in XENPV now and XENPV does not actually need the
> preconstructed identity page tables directly, they can be filled in
> xen_setup_kernel_pagetable(). Therefore, build the identity mapping page
> table dynamically to remove the preconstructed page tables and make the
> code cleaner.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---
>  arch/x86/include/asm/pgtable_64.h |  2 --
>  arch/x86/kernel/head_64.S         | 28 ----------------------------
>  arch/x86/xen/mmu_pv.c             |  9 +++++++++
>  3 files changed, 9 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
> index f06e5d6a2747..ce45882ccd07 100644
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -19,10 +19,8 @@
>  extern p4d_t level4_kernel_pgt[512];
>  extern p4d_t level4_ident_pgt[512];
>  extern pud_t level3_kernel_pgt[512];
> -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 * FIXMAP_PMD_NUM];
>  extern pgd_t init_top_pgt[];
>  
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 21816b48537c..85d4a5094f6b 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -616,38 +616,10 @@ SYM_DATA(early_recursion_flag, .long 0)
>  
>  	.data
>  
> -#if defined(CONFIG_XEN_PV) || defined(CONFIG_PVH)
> -SYM_DATA_START_PTI_ALIGNED(init_top_pgt)
> -	.quad   level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
> -	.org    init_top_pgt + L4_PAGE_OFFSET*8, 0
> -	.quad   level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
> -	.org    init_top_pgt + L4_START_KERNEL*8, 0
> -	/* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
> -	.quad   level3_kernel_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
> -	.fill	PTI_USER_PGD_FILL,8,0
> -SYM_DATA_END(init_top_pgt)
> -
> -SYM_DATA_START_PAGE_ALIGNED(level3_ident_pgt)
> -	.quad	level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
> -	.fill	511, 8, 0
> -SYM_DATA_END(level3_ident_pgt)
> -SYM_DATA_START_PAGE_ALIGNED(level2_ident_pgt)
> -	/*
> -	 * Since I easily can, map the first 1G.
> -	 * Don't set NX because code runs from these pages.
> -	 *
> -	 * Note: This sets _PAGE_GLOBAL despite whether
> -	 * the CPU supports it or it is enabled.  But,
> -	 * the CPU should ignore the bit.
> -	 */
> -	PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
> -SYM_DATA_END(level2_ident_pgt)
> -#else
>  SYM_DATA_START_PTI_ALIGNED(init_top_pgt)
>  	.fill	512,8,0
>  	.fill	PTI_USER_PGD_FILL,8,0
>  SYM_DATA_END(init_top_pgt)
> -#endif
>  
>  SYM_DATA_START_PAGE_ALIGNED(level4_kernel_pgt)
>  	.fill	511,8,0
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 9fa00c4a8858..7d77c233002b 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -105,6 +105,9 @@ pte_t xen_make_pte_init(pteval_t pte);
>  static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
>  #endif
>  
> +static pud_t level3_ident_pgt[PTRS_PER_PUD] __page_aligned_bss;
> +static pmd_t level2_ident_pgt[PTRS_PER_PMD] __page_aligned_bss;
> +
>  /*
>   * Protects atomic reservation decrease/increase against concurrent increases.
>   * Also protects non-atomic updates of current_pages and balloon lists.
> @@ -1773,6 +1776,12 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
>  	/* Zap identity mapping */
>  	init_top_pgt[0] = __pgd(0);
>  
> +	init_top_pgt[pgd_index(__PAGE_OFFSET_BASE_L4)].pgd =
> +		__pa_symbol(level3_ident_pgt) + _KERNPG_TABLE_NOENC;
> +	init_top_pgt[pgd_index(__START_KERNEL_map)].pgd =
> +		__pa_symbol(level3_kernel_pgt) + _PAGE_TABLE_NOENC;
> +	level3_ident_pgt[0].pud = __pa_symbol(level2_ident_pgt) + _KERNPG_TABLE_NOENC;
> +
>  	/* Pre-constructed entries are in pfn, so convert to mfn */
>  	/* L4[273] -> level3_ident_pgt  */
>  	/* L4[511] -> level3_kernel_pgt */


I obviously am very much in agreement with the arch/x86/kernel/head_64.S hunk,
removing all that gunk.

Provided there's no some gotcha in that change which Xen people would
hopefully point out and the pagetables look the same:

Acked-by: Borislav Petkov (AMD) <bp@alien8.de>

If people want me to take it through tip, just holler.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH] x86/xen: Build identity mapping page tables dynamically for XENPV

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 730 bytes --]

On 22.01.26 11:06, Hou Wenlong wrote:
> After commit 47ffe0578aee ("x86/pvh: Add 64bit relocation page tables"),
> the PVH entry uses a new set of page tables instead of the
> preconstructed page tables in head64.S. Since those preconstructed page
> tables are only used in XENPV now and XENPV does not actually need the
> preconstructed identity page tables directly, they can be filled in
> xen_setup_kernel_pagetable(). Therefore, build the identity mapping page
> table dynamically to remove the preconstructed page tables and make the
> code cleaner.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>

Reviewed-by: Juergen Gross <jgross@suse.com>

I'll take this one via the Xen tree.


Juergen

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