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[PATCH v5 1/4] xen/mm: remove aliasing of PGC_need_scrub over PGC_allocated

From: Roger Pau Monne <roger.pau@xxxxxxxxxx>

Date: Fri, 30 Jan 2026 15:57:23 +0100

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Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Timothy Pearson <tpearson@xxxxxxxxxxxxxxxxxxxxx>, Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Delivery-date: Fri, 30 Jan 2026 15:00:11 +0000

List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

Future changes will care about the state of the PGC_need_scrub flag even when pages have the PGC_allocated set, and hence it's no longer possible to alias both values. Also introduce PGC_need_scrub to the set of preserved flags, so it's not dropped by assign_pages(). No functional change intended, albeit the page counter on x86 looses a bit. Suggested-by: Jan Beulich <jbeulich@xxxxxxxx> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> --- Changes since v4: - New in this version. --- The PGC space on arches different than x86 is not compact, so I've just used a hole in those. --- xen/arch/arm/include/asm/mm.h | 10 +++------- xen/arch/ppc/include/asm/mm.h | 10 +++------- xen/arch/riscv/include/asm/mm.h | 10 +++------- xen/arch/x86/include/asm/mm.h | 18 +++++++----------- xen/common/page_alloc.c | 2 +- 5 files changed, 17 insertions(+), 33 deletions(-) diff --git a/xen/arch/arm/include/asm/mm.h b/xen/arch/arm/include/asm/mm.h index ec2d2dc5372a..72a692862420 100644 --- a/xen/arch/arm/include/asm/mm.h +++ b/xen/arch/arm/include/asm/mm.h @@ -144,6 +144,9 @@ struct page_info #define _PGC_colored PG_shift(4) #define PGC_colored PG_mask(1, 4) #endif +/* Page needs to be scrubbed. */ +#define _PGC_need_scrub PG_shift(5) +#define PGC_need_scrub PG_mask(1, 5) /* ... */ /* Page is broken? */ #define _PGC_broken PG_shift(7) @@ -163,13 +166,6 @@ struct page_info #define PGC_count_width PG_shift(10) #define PGC_count_mask ((1UL<<PGC_count_width)-1) -/* - * Page needs to be scrubbed. Since this bit can only be set on a page that is - * free (i.e. in PGC_state_free) we can reuse PGC_allocated bit. - */ -#define _PGC_need_scrub _PGC_allocated -#define PGC_need_scrub PGC_allocated - #if defined(CONFIG_ARM_64) || defined(CONFIG_MPU) #define is_xen_heap_page(page) ((page)->count_info & PGC_xen_heap) #define is_xen_heap_mfn(mfn) \ diff --git a/xen/arch/ppc/include/asm/mm.h b/xen/arch/ppc/include/asm/mm.h index 91c405876bd0..402d06bdaa9f 100644 --- a/xen/arch/ppc/include/asm/mm.h +++ b/xen/arch/ppc/include/asm/mm.h @@ -57,6 +57,9 @@ static inline struct page_info *virt_to_page(const void *v) /* Page is Xen heap? */ #define _PGC_xen_heap PG_shift(2) #define PGC_xen_heap PG_mask(1, 2) +/* Page needs to be scrubbed. */ +#define _PGC_need_scrub PG_shift(3) +#define PGC_need_scrub PG_mask(1, 3) /* Page is broken? */ #define _PGC_broken PG_shift(7) #define PGC_broken PG_mask(1, 7) @@ -75,13 +78,6 @@ static inline struct page_info *virt_to_page(const void *v) #define PGC_count_width PG_shift(10) #define PGC_count_mask ((1UL<<PGC_count_width)-1) -/* - * Page needs to be scrubbed. Since this bit can only be set on a page that is - * free (i.e. in PGC_state_free) we can reuse PGC_allocated bit. - */ -#define _PGC_need_scrub _PGC_allocated -#define PGC_need_scrub PGC_allocated - #define is_xen_heap_page(page) ((page)->count_info & PGC_xen_heap) #define is_xen_heap_mfn(mfn) \ (mfn_valid(mfn) && is_xen_heap_page(mfn_to_page(mfn))) diff --git a/xen/arch/riscv/include/asm/mm.h b/xen/arch/riscv/include/asm/mm.h index a005d0247a6f..9e28c2495462 100644 --- a/xen/arch/riscv/include/asm/mm.h +++ b/xen/arch/riscv/include/asm/mm.h @@ -273,13 +273,6 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr) #define PGT_count_width PG_shift(2) #define PGT_count_mask ((1UL << PGT_count_width) - 1) -/* - * Page needs to be scrubbed. Since this bit can only be set on a page that is - * free (i.e. in PGC_state_free) we can reuse PGC_allocated bit. - */ -#define _PGC_need_scrub _PGC_allocated -#define PGC_need_scrub PGC_allocated - /* Cleared when the owning guest 'frees' this page. */ #define _PGC_allocated PG_shift(1) #define PGC_allocated PG_mask(1, 1) @@ -293,6 +286,9 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr) #else #define PGC_static 0 #endif +/* Page needs to be scrubbed. */ +#define _PGC_need_scrub PG_shift(4) +#define PGC_need_scrub PG_mask(1, 4) /* Page is broken? */ #define _PGC_broken PG_shift(7) #define PGC_broken PG_mask(1, 7) diff --git a/xen/arch/x86/include/asm/mm.h b/xen/arch/x86/include/asm/mm.h index 419fa17a4373..06c20ab8de33 100644 --- a/xen/arch/x86/include/asm/mm.h +++ b/xen/arch/x86/include/asm/mm.h @@ -83,29 +83,25 @@ #define PGC_state_offlined PG_mask(2, 6) #define PGC_state_free PG_mask(3, 6) #define page_state_is(pg, st) (((pg)->count_info&PGC_state) == PGC_state_##st) +/* Page needs to be scrubbed. */ +#define _PGC_need_scrub PG_shift(7) +#define PGC_need_scrub PG_mask(1, 7) #ifdef CONFIG_SHADOW_PAGING /* Set when a page table page has been shadowed. */ -#define _PGC_shadowed_pt PG_shift(7) -#define PGC_shadowed_pt PG_mask(1, 7) +#define _PGC_shadowed_pt PG_shift(8) +#define PGC_shadowed_pt PG_mask(1, 8) #else #define PGC_shadowed_pt 0 #endif /* Count of references to this frame. */ #if PGC_shadowed_pt -#define PGC_count_width PG_shift(7) +#define PGC_count_width PG_shift(8) #else -#define PGC_count_width PG_shift(6) +#define PGC_count_width PG_shift(7) #endif #define PGC_count_mask ((1UL<<PGC_count_width)-1) -/* - * Page needs to be scrubbed. Since this bit can only be set on a page that is - * free (i.e. in PGC_state_free) we can reuse PGC_allocated bit. - */ -#define _PGC_need_scrub _PGC_allocated -#define PGC_need_scrub PGC_allocated - #ifndef CONFIG_BIGMEM /* * This definition is solely for the use in struct page_info (and diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c index 2efc11ce095f..2ee249ac365a 100644 --- a/xen/common/page_alloc.c +++ b/xen/common/page_alloc.c @@ -170,7 +170,7 @@ /* * Flags that are preserved in assign_pages() (and only there) */ -#define PGC_preserved (PGC_extra | PGC_static | PGC_colored) +#define PGC_preserved (PGC_extra | PGC_static | PGC_colored | PGC_need_scrub) #ifndef PGT_TYPE_INFO_INITIALIZER #define PGT_TYPE_INFO_INITIALIZER 0 -- 2.51.0

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