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Subject[PATCH 4/4] x86/mm: Introduce 'no5lvl' kernel parameter
Date
The kernel parameter allows to force kernel to use 4-level paging even
if hardware and kernel support 5-level paging.

The option may be useful to workaround regressions related to 5-level
paging.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/x86/boot/compressed/cmdline.c | 2 +-
arch/x86/boot/compressed/head_64.S | 1 +
arch/x86/boot/compressed/pgtable_64.c | 12 ++++++++++--
arch/x86/kernel/cpu/common.c | 6 ++++++
arch/x86/kernel/head64.c | 10 ++++++----
6 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 11fc28ecdb6d..364a33c1534d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2600,6 +2600,9 @@
emulation library even if a 387 maths coprocessor
is present.

+ no5lvl [X86-64] Disable 5-level paging mode. Forces
+ kernel to use 4-level paging instead.
+
no_console_suspend
[HW] Never suspend the console
Disable suspending of consoles during suspend and
diff --git a/arch/x86/boot/compressed/cmdline.c b/arch/x86/boot/compressed/cmdline.c
index 0cb325734cfb..af6cda0b7900 100644
--- a/arch/x86/boot/compressed/cmdline.c
+++ b/arch/x86/boot/compressed/cmdline.c
@@ -1,7 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
#include "misc.h"

-#if CONFIG_EARLY_PRINTK || CONFIG_RANDOMIZE_BASE
+#if CONFIG_EARLY_PRINTK || CONFIG_RANDOMIZE_BASE || CONFIG_X86_5LEVEL

static unsigned long fs;
static inline void set_fs(unsigned long seg)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 1babefd9ddfc..763793349cdd 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -365,6 +365,7 @@ ENTRY(startup_64)
* this function call.
*/
pushq %rsi
+ movq %rsi, %rdi /* real mode address */
call paging_prepare
popq %rsi

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 23707e1da1ff..8c5107545251 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -31,16 +31,23 @@ static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
*/
unsigned long *trampoline_32bit __section(.data);

-struct paging_config paging_prepare(void)
+extern struct boot_params *boot_params;
+int cmdline_find_option_bool(const char *option);
+
+struct paging_config paging_prepare(void *rmode)
{
struct paging_config paging_config = {};
unsigned long bios_start, ebda_start;

+ /* Initialize boot_params. Required for cmdline_find_option_bool(). */
+ boot_params = rmode;
+
/*
* Check if LA57 is desired and supported.
*
- * There are two parts to the check:
+ * There are several parts to the check:
* - if the kernel supports 5-level paging: CONFIG_X86_5LEVEL=y
+ * - if user asked to disable 5-level paging: no5lvl in cmdline
* - if the machine supports 5-level paging:
* + CPUID leaf 7 is supported
* + the leaf has the feature bit set
@@ -48,6 +55,7 @@ struct paging_config paging_prepare(void)
* That's substitute for boot_cpu_has() in early boot code.
*/
if (IS_ENABLED(CONFIG_X86_5LEVEL) &&
+ !cmdline_find_option_bool("no5lvl") &&
native_cpuid_eax(0) >= 7 &&
(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
paging_config.l5_required = 1;
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ce243f7d2d4e..1e91ec6293de 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1008,6 +1008,12 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
*/
setup_clear_cpu_cap(X86_FEATURE_PCID);
#endif
+
+#ifdef CONFIG_X86_5LEVEL
+ /* Clear the 5-level paging feature if user asked for 'no5lvl' */
+ if (!__pgtable_l5_enabled)
+ setup_clear_cpu_cap(X86_FEATURE_LA57);
+#endif
}

void __init early_cpu_init(void)
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 0c408f8c4ed4..8ca65d35b93a 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -82,10 +82,12 @@ static unsigned int __head *fixup_int(void *ptr, unsigned long physaddr)

static bool __head check_la57_support(unsigned long physaddr)
{
- if (native_cpuid_eax(0) < 7)
- return false;
-
- if (!(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31))))
+ /*
+ * 5-level paging is detected and enabled on kernel decomression
+ * stage. Back off if 5-level paging mode has not yet enabled by
+ * this point.
+ */
+ if (!(native_read_cr4() & X86_CR4_LA57))
return false;

*fixup_int(&pgtable_l5_enabled, physaddr) = 1;
--
2.17.0
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