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Subject[PATCH v2 2/5] x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly
Date
The initialization process for trampoline_cr4_features and
mmu_cr4_features was confusing. The intent is for mmu_cr4_features
and *trampoline_cr4_features to stay in sync, but
trampoline_cr4_features is NULL until setup_real_mode() runs. The
old code synchronized *trampoline_cr4_features *twice*, once in
setup_real_mode() and once in setup_arch(). It also initialized
mmu_cr4_features in setup_real_mode(), which causes the actual value
of mmu_cr4_features to potentially depend on when setup_real_mode()
is called.

With this patch, mmu_cr4_features is initialized directly in
setup_arch(), and *trampoline_cr4_features is synchronized to
mmu_cr4_features when the trampoline is set up.

After this patch, it should be safe to defer setup_real_mode().

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/kernel/setup.c | 17 ++++++++++-------
arch/x86/realmode/init.c | 3 ++-
2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 082ec5171ddc..47006b28cac4 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1127,6 +1127,16 @@ void __init setup_arch(char **cmdline_p)

early_trap_pf_init();

+ /*
+ * Update mmu_cr4_features (and, indirectly, trampoline_cr4_features)
+ * with the current CR4 value. This may not be necessary, but
+ * auditing all the early-boot CR4 manipulation would be needed to
+ * rule it out.
+ */
+ if (boot_cpu_data.cpuid_level >= 0)
+ /* A CPU has %cr4 if and only if it has CPUID. */
+ mmu_cr4_features = __read_cr4();
+
setup_real_mode();

memblock_set_current_limit(get_max_mapped());
@@ -1176,13 +1186,6 @@ void __init setup_arch(char **cmdline_p)

kasan_init();

- if (boot_cpu_data.cpuid_level >= 0) {
- /* A CPU has %cr4 if and only if it has CPUID */
- mmu_cr4_features = __read_cr4();
- if (trampoline_cr4_features)
- *trampoline_cr4_features = mmu_cr4_features;
- }
-
#ifdef CONFIG_X86_32
/* sync back kernel address range */
clone_pgd_range(initial_page_table + KERNEL_PGD_BOUNDARY,
diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index 705e3fffb4a1..c5bdc4e473e7 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -4,6 +4,7 @@
#include <asm/cacheflush.h>
#include <asm/pgtable.h>
#include <asm/realmode.h>
+#include <asm/tlbflush.h>

struct real_mode_header *real_mode_header;
u32 *trampoline_cr4_features;
@@ -84,7 +85,7 @@ void __init setup_real_mode(void)

trampoline_header->start = (u64) secondary_startup_64;
trampoline_cr4_features = &trampoline_header->cr4;
- *trampoline_cr4_features = __read_cr4();
+ *trampoline_cr4_features = mmu_cr4_features;

trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd);
trampoline_pgd[0] = trampoline_pgd_entry.pgd;
--
2.7.4
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