From: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
To: x86@kernel.org
Cc: Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ard Biesheuvel <ardb@kernel.org>,
Nathan Chancellor <nathan@kernel.org>, Rong Xu <xur@google.com>,
Kees Cook <kees@kernel.org>,
linux-kernel@vger.kernel.org,
Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Subject: [PATCH v7] x86/mm/64: free the gap between BSS_MAIN and BSS_DECRYPTED
Date: Fri, 28 Aug 2026 20:32:45 -0400 [thread overview]
Message-ID: <20260829003245.2334947-1-hamzamahfooz@linux.microsoft.com> (raw)
This region is unused after mark_rodata_ro() and takes up, up to 2 MiB
of memory. So, free it in mark_rodata_ro().
Tested on a CoCo VM running on an AMD EPYC 7763v which generated the
following print:
Freeing unused kernel image (bss_decrypted gap) memory: 184K
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
arch/x86/include/asm/sections.h | 1 +
arch/x86/kernel/vmlinux.lds.S | 8 +++++++-
arch/x86/mm/init_64.c | 2 ++
3 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/sections.h b/arch/x86/include/asm/sections.h
index 30e8ee7006f9..49ef1428aa72 100644
--- a/arch/x86/include/asm/sections.h
+++ b/arch/x86/include/asm/sections.h
@@ -8,6 +8,7 @@
extern char __relocate_kernel_start[], __relocate_kernel_end[];
extern char __brk_base[], __brk_limit[];
extern char __end_rodata_aligned[];
+extern char __start_bss_decrypted_gap[], __end_bss_decrypted_gap[];
#if defined(CONFIG_X86_64)
extern char __end_rodata_hpage_align[];
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 2438b89a4620..825381b1ed33 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -94,8 +94,11 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
* decrypted to avoid exposing more than we wish.
*/
#define BSS_DECRYPTED \
+ . = ALIGN(PAGE_SIZE); \
+ __start_bss_decrypted_gap = .; \
. = ALIGN(PMD_SIZE); \
__start_bss_decrypted = .; \
+ __end_bss_decrypted_gap = .; \
__pi___start_bss_decrypted = .; \
*(.bss..decrypted); \
. = ALIGN(PAGE_SIZE); \
@@ -105,7 +108,10 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
__pi___end_bss_decrypted = .; \
#else
-#define BSS_DECRYPTED
+#define BSS_DECRYPTED \
+ . = ALIGN(PAGE_SIZE); \
+ __start_bss_decrypted_gap = .; \
+ __end_bss_decrypted_gap = .;
#endif
#if defined(CONFIG_X86_64) && defined(CONFIG_KEXEC_CORE)
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 70e682180291..d2e1f087ef31 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1400,6 +1400,8 @@ void mark_rodata_ro(void)
(void *)text_end, (void *)rodata_start);
free_kernel_image_pages("unused kernel image (rodata/data gap)",
(void *)rodata_end, (void *)_sdata);
+ free_kernel_image_pages("unused kernel image (bss_decrypted gap)",
+ __start_bss_decrypted_gap, __end_bss_decrypted_gap);
}
/*
--
2.55.0
next reply other threads:[~2026-08-29 0:35 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-29 0:32 Hamza Mahfooz [this message]
2026-09-09 3:47 ` [PATCH v7] x86/mm/64: free the gap between BSS_MAIN and BSS_DECRYPTED Borislav Petkov
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