[PATCHv2 3/4] x86/boot/compressed/64: Introduce place_trampoline()

[Kirill A. Shutemov]

If bootloader enables 64-bit mode with 4-level paging, we might need to
switch over to 5-level paging. The switching requires disabling paging.
It works fine if kernel itself is loaded below 4G.

If bootloader put the kernel above 4G (not sure if anybody does this),
we would loose control as soon as paging is disabled as code becomes
unreachable.

To handle the situation, we need a trampoline in lower memory that would
take care about switching on 5-level paging.

Apart from trampoline itself we also need place to store top level page
table in lower memory as we don't have a way to load 64-bit value into
CR3 from 32-bit mode. We only really need 8-bytes there as we only use
the very first entry of the page table. But we allocate whole page
anyway. We cannot have the code in the same because, there's hazard that
a CPU would read page table speculatively and get confused seeing
garbage.

This patch introduces paging_prepare() that check if we need to enable
5-level paging and then finds right spot in lower memory for trampoline,
copies trampoline code there and setups new top level page table for
5-level paging.

At this point we do all the preparation, but not yet use trampoline.
It will be done in following patch.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S    | 54 ++++++++++++++++-------------
 arch/x86/boot/compressed/pgtable.h    | 18 ++++++++++
 arch/x86/boot/compressed/pgtable_64.c | 65 +++++++++++++++++++++++++++++------
 3 files changed, 103 insertions(+), 34 deletions(-)
 create mode 100644 arch/x86/boot/compressed/pgtable.h

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index fc313e29fe2c..33a47d5c6445 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -304,33 +304,45 @@ ENTRY(startup_64)
 	/* Set up the stack */
 	leaq	boot_stack_end(%rbx), %rsp
 
-#ifdef CONFIG_X86_5LEVEL
-	/*
-	 * Check if we need to enable 5-level paging.
-	 * RSI holds real mode data and need to be preserved across
-	 * a function call.
-	 */
-	pushq	%rsi
-	call	l5_paging_required
-	popq	%rsi
-
-	/* If l5_paging_required() returned zero, we're done here. */
-	cmpq	$0, %rax
-	je	lvl5
-
 	/*
 	 * At this point we are in long mode with 4-level paging enabled,
-	 * but we want to enable 5-level paging.
+	 * but we might want to enable 5-level paging.
 	 *
 	 * The problem is that we cannot do it directly. Setting LA57 in
 	 * long mode would trigger #GP. So we need to switch off long mode
 	 * first.
 	 *
-	 * NOTE: This is not going to work if bootloader put us above 4G
-	 * limit.
+	 * We also need trampoline in lower memory to switch from 4- to 5-level
+	 * paging for cases when bootloader put kernel above 4G, but didn't
+	 * enable 5-level paging for us.
+	 *
+	 * For trampoline, we have to have top page table in lower memory as we
+	 * don't have a way to load 64-bit value into CR3 from 32-bit mode.
+	 *
+	 * We go though trampoline even if we don't have to: if we're already
+	 * in 5-level paging mode or if we don't need to switch to it. This way
+	 * the trampoline code gets tested not only in special rare case, but
+	 * on every boot.
+	 */
+
+	/*
+	 * paging_prepare() would setup trampoline and check if we need to
+	 * enable 5-level paging.
+	 *
+	 * Address of trampoline is rerurned in RAX. The bit 0 is used to
+	 * encode if we need to enabled 5-level paging.
 	 *
-	 * The first step is go into compatibility mode.
+	 * RSI holds real mode data and need to be preserved across
+	 * a function call.
 	 */
+	pushq	%rsi
+	call	paging_prepare
+	popq	%rsi
+	movq	%rax, %rcx
+	andq	$(~1UL), %rcx
+
+	testq	$1, %rax
+	jz	lvl5
 
 	/* Clear additional page table */
 	leaq	lvl5_pgtable(%rbx), %rdi
@@ -352,7 +364,6 @@ ENTRY(startup_64)
 	pushq	%rax
 	lretq
 lvl5:
-#endif
 
 	/* Zero EFLAGS */
 	pushq	$0
@@ -490,7 +501,7 @@ relocated:
 	jmp	*%rax
 
 	.code32
-#ifdef CONFIG_X86_5LEVEL
+ENTRY(trampoline_32bit_src)
 compatible_mode:
 	/* Setup data and stack segments */
 	movl	$__KERNEL_DS, %eax
@@ -526,7 +537,6 @@ compatible_mode:
 	movl	%eax, %cr0
 
 	lret
-#endif
 
 no_longmode:
 	/* This isn't an x86-64 CPU so hang */
@@ -585,7 +595,5 @@ boot_stack_end:
 	.balign 4096
 pgtable:
 	.fill BOOT_PGT_SIZE, 1, 0
-#ifdef CONFIG_X86_5LEVEL
 lvl5_pgtable:
 	.fill PAGE_SIZE, 1, 0
-#endif
diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
new file mode 100644
index 000000000000..0261d4ab62e6
--- /dev/null
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -0,0 +1,18 @@
+#ifndef BOOT_COMPRESSED_PAGETABLE_H
+#define BOOT_COMPRESSED_PAGETABLE_H
+
+#define TRAMPOLINE_32BIT_SIZE		(2 * PAGE_SIZE)
+
+#define TRAMPOLINE_32BIT_PGTABLE_OFF	0
+
+#define TRAMPOLINE_32BIT_CODE_OFF	PAGE_SIZE
+#define TRAMPOLINE_32BIT_CODE_SIZE	0x50
+
+#define TRAMPOLINE_32BIT_STACK_END	TRAMPOLINE_32BIT_SIZE
+
+#ifndef __ASSEMBLER__
+
+extern void (*trampoline_32bit_src)(void *return_ptr);
+
+#endif /* __ASSEMBLER__ */
+#endif /* BOOT_COMPRESSED_PAGETABLE_H */
diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index eed3a2c3b577..a2ab6b9cf258 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -1,18 +1,61 @@
 #include <asm/processor.h>
+#include "pgtable.h"
+#include "../string.h"
 
-int l5_paging_required(void)
+#define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
+#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
+
+unsigned long paging_prepare(void)
 {
-	/* Check i leaf 7 is supported. */
-	if (native_cpuid_eax(0) < 7)
-		return 0;
+	unsigned long bios_start, ebda_start, trampoline_start, *trampoline;
+	int l5_required = 0;
+
+	/* Check if la57 is desired and supported */
+	if (IS_ENABLED(CONFIG_X86_5LEVEL) && native_cpuid_eax(0) >= 7 &&
+			(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31))))
+		l5_required = 1;
+
+	/*
+	 * Find suitable spot for trampoline.
+	 * Based on reserve_bios_regions().
+	 */
+
+	ebda_start = *(unsigned short *)0x40e << 4;
+	bios_start = *(unsigned short *)0x413 << 10;
+
+	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
+		bios_start = BIOS_START_MAX;
+
+	if (ebda_start > BIOS_START_MIN && ebda_start < bios_start)
+		bios_start = ebda_start;
+
+	/* Place trampoline below end of low memory, aligned to 4k */
+	trampoline_start = bios_start - TRAMPOLINE_32BIT_SIZE;
+	trampoline_start = round_down(trampoline_start, PAGE_SIZE);
+
+	trampoline = (unsigned long *)trampoline_start;
+
+	/* Clear trampoline memory first */
+	memset(trampoline, 0, TRAMPOLINE_32BIT_SIZE);
 
-	/* Check if la57 is supported. */
-	if (!(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31))))
-		return 0;
+	/* Copy trampoline code in place */
+	memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFF / sizeof(unsigned long),
+			&trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
 
-	/* Check if 5-level paging has already been enabled. */
-	if (native_read_cr4() & X86_CR4_LA57)
-		return 0;
+	if (l5_required) {
+		/*
+		 * For 5-level paging setup current CR3 as the first and the
+		 * only entry in a new top level page table.
+		 */
+		trampoline[0] = __read_cr3() + _PAGE_TABLE_NOENC;
+	} else {
+		/*
+		 * For 4-level paging, copy current top-level page table.
+		 * It might be above 4G and be unaccessible from 32-bit mode.
+		 */
+		memcpy(trampoline, (void *)__read_cr3(), PAGE_SIZE);
+	}
 
-	return 1;
+	/* Bit 0 is used to encode if 5-level paging is required */
+	return trampoline_start | l5_required;
 }
-- 
2.14.2