[PATCH v8 12/23] x86/decompressor: Call trampoline directly from C code

Ard Biesheuvel
2023-08-02T15:50:52Z

Instead of returning to the asm calling code to invoke the trampoline,
call it straight from the C code that sets it up. That way, the struct
return type is no longer needed for returning two values, and the call
can be made conditional more cleanly in a subsequent patch.

This means that all callee save 64-bit registers need to be preserved
and restored, as their contents may not survive the legacy mode switch.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/head_64.S | 31 ++++++++-----------
arch/x86/boot/compressed/pgtable_64.c | 32 ++++++++------------
2 files changed, 26 insertions(+), 37 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 37fd7b7d683d696c..cd6e3e175389aa6b 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -430,25 +430,14 @@ SYM_CODE_START(startup_64)
#endif

/*
- * paging_prepare() sets up the trampoline and checks if we need to
- * enable 5-level paging.
- *
- * paging_prepare() returns a two-quadword structure which lands
- * into RDX:RAX:
- * - Address of the trampoline is returned in RAX.
- * - Non zero RDX means trampoline needs to enable 5-level
- * paging.
+ * configure_5level_paging() updates the number of paging levels using
+ * a trampoline in 32-bit addressable memory if the current number does
+ * not match the desired number.
*
* Pass the boot_params pointer as the first argument.
*/
movq %r15, %rdi
- call paging_prepare
-
- /* Pass the trampoline address and boolean flag as args #1 and #2 */
- movq %rax, %rdi
- movq %rdx, %rsi
- leaq TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
- call *%rax
+ call configure_5level_paging

/*
* cleanup_trampoline() would restore trampoline memory.
@@ -543,11 +532,14 @@ SYM_FUNC_END(.Lrelocated)
.section ".rodata", "a", @progbits
SYM_CODE_START(trampoline_32bit_src)
/*
- * Preserve live 64-bit registers on the stack: this is necessary
- * because the architecture does not guarantee that GPRs will retain
- * their full 64-bit values across a 32-bit mode switch.
+ * Preserve callee save 64-bit registers on the stack: this is
+ * necessary because the architecture does not guarantee that GPRs will
+ * retain their full 64-bit values across a 32-bit mode switch.
*/
pushq %r15
+ pushq %r14
+ pushq %r13
+ pushq %r12
pushq %rbp
pushq %rbx

@@ -574,6 +566,9 @@ SYM_CODE_START(trampoline_32bit_src)
/* Restore the preserved 64-bit registers */
popq %rbx
popq %rbp
+ popq %r12
+ popq %r13
+ popq %r14
popq %r15
retq

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 5198a05aefa8d14a..f9cc86b2ee55ca80 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -16,11 +16,6 @@ unsigned int __section(".data") pgdir_shift = 39;
unsigned int __section(".data") ptrs_per_p4d = 1;
#endif

-struct paging_config {
- unsigned long trampoline_start;
- unsigned long l5_required;
-};
-
/* Buffer to preserve trampoline memory */
static char trampoline_save[TRAMPOLINE_32BIT_SIZE];

@@ -29,7 +24,7 @@ static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
* purposes.
*
* Avoid putting the pointer into .bss as it will be cleared between
- * paging_prepare() and extract_kernel().
+ * configure_5level_paging() and extract_kernel().
*/
unsigned long *trampoline_32bit __section(".data");

@@ -106,13 +101,13 @@ static unsigned long find_trampoline_placement(void)
return bios_start - TRAMPOLINE_32BIT_SIZE;
}

-struct paging_config paging_prepare(void *rmode)
+asmlinkage void configure_5level_paging(struct boot_params *bp)
{
- struct paging_config paging_config = {};
- void *tramp_code;
+ void (*toggle_la57)(void *trampoline, bool enable_5lvl);
+ bool l5_required = false;

/* Initialize boot_params. Required for cmdline_find_option_bool(). */
- boot_params = rmode;
+ boot_params = bp;

/*
* Check if LA57 is desired and supported.
@@ -130,7 +125,7 @@ struct paging_config paging_prepare(void *rmode)
!cmdline_find_option_bool("no5lvl") &&
native_cpuid_eax(0) >= 7 &&
(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
- paging_config.l5_required = 1;
+ l5_required = true;

/* Initialize variables for 5-level paging */
__pgtable_l5_enabled = 1;
@@ -138,9 +133,7 @@ struct paging_config paging_prepare(void *rmode)
ptrs_per_p4d = 512;
}

- paging_config.trampoline_start = find_trampoline_placement();
-
- trampoline_32bit = (unsigned long *)paging_config.trampoline_start;
+ trampoline_32bit = (unsigned long *)find_trampoline_placement();

/* Preserve trampoline memory */
memcpy(trampoline_save, trampoline_32bit, TRAMPOLINE_32BIT_SIZE);
@@ -149,7 +142,7 @@ struct paging_config paging_prepare(void *rmode)
memset(trampoline_32bit, 0, TRAMPOLINE_32BIT_SIZE);

/* Copy trampoline code in place */
- tramp_code = memcpy(trampoline_32bit +
+ toggle_la57 = memcpy(trampoline_32bit +
TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
&trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);

@@ -159,7 +152,8 @@ struct paging_config paging_prepare(void *rmode)
* immediate absolute address, which needs to be adjusted based on the
* placement of the trampoline.
*/
- *(u32 *)(tramp_code + trampoline_ljmp_imm_offset) += (unsigned long)tramp_code;
+ *(u32 *)((u8 *)toggle_la57 + trampoline_ljmp_imm_offset) +=
+ (unsigned long)toggle_la57;

/*
* The code below prepares page table in trampoline memory.
@@ -175,10 +169,10 @@ struct paging_config paging_prepare(void *rmode)
* We are not going to use the page table in trampoline memory if we
* are already in the desired paging mode.
*/
- if (paging_config.l5_required == !!(native_read_cr4() & X86_CR4_LA57))
+ if (l5_required == !!(native_read_cr4() & X86_CR4_LA57))
goto out;

- if (paging_config.l5_required) {
+ if (l5_required) {
/*
* For 4- to 5-level paging transition, set up current CR3 as
* the first and the only entry in a new top-level page table.
@@ -201,7 +195,7 @@ struct paging_config paging_prepare(void *rmode)
}

out:
- return paging_config;
+ toggle_la57(trampoline_32bit, l5_required);
}

void cleanup_trampoline(void *pgtable)
--
2.39.2