Messages in this thread | | | Date | Fri, 31 Mar 2023 18:00:35 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86/boot: Clean up handling of boot_params pointer |
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On March 31, 2023 11:28:39 AM PDT, Brian Gerst <brgerst@gmail.com> wrote: >On entry from the bootloader, RSI contains the pointer to the >boot_params data structure. Since the RSI register can be clobbered >when calling C functions, it is saved and restored around every call. >Instead, move it to the R12 register, which is preserved across calls. > >Signed-off-by: Brian Gerst <brgerst@gmail.com> >--- > arch/x86/kernel/head_64.S | 29 ++++++++++------------------- > 1 file changed, 10 insertions(+), 19 deletions(-) > >diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S >index a5df3e994f04..0d130ca2e0a3 100644 >--- a/arch/x86/kernel/head_64.S >+++ b/arch/x86/kernel/head_64.S >@@ -49,8 +49,6 @@ SYM_CODE_START_NOALIGN(startup_64) > * for us. These identity mapped page tables map all of the > * kernel pages and possibly all of memory. > * >- * %rsi holds a physical pointer to real_mode_data. >- * > * We come here either directly from a 64bit bootloader, or from > * arch/x86/boot/compressed/head_64.S. > * >@@ -61,6 +59,12 @@ SYM_CODE_START_NOALIGN(startup_64) > * tables and then reload them. > */ > >+ /* >+ * RSI holds a physical pointer to real_mode_data. Move it to R12, >+ * which is preserved across C function calls. >+ */ >+ movq %rsi, %r12 >+ > /* Set up the stack for verify_cpu() */ > leaq (__end_init_task - PTREGS_SIZE)(%rip), %rsp > >@@ -73,9 +77,7 @@ SYM_CODE_START_NOALIGN(startup_64) > shrq $32, %rdx > wrmsr > >- pushq %rsi > call startup_64_setup_env >- popq %rsi > > #ifdef CONFIG_AMD_MEM_ENCRYPT > /* >@@ -84,10 +86,8 @@ SYM_CODE_START_NOALIGN(startup_64) > * which needs to be done before any CPUID instructions are executed in > * subsequent code. > */ >- movq %rsi, %rdi >- pushq %rsi >+ movq %r12, %rdi > call sme_enable >- popq %rsi > #endif > > /* Now switch to __KERNEL_CS so IRET works reliably */ >@@ -109,9 +109,8 @@ SYM_CODE_START_NOALIGN(startup_64) > * programmed into CR3. > */ > leaq _text(%rip), %rdi >- pushq %rsi >+ movq %r12, %rsi > call __startup_64 >- popq %rsi > > /* Form the CR3 value being sure to include the CR3 modifier */ > addq $(early_top_pgt - __START_KERNEL_map), %rax >@@ -125,8 +124,6 @@ SYM_CODE_START(secondary_startup_64) > * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0, > * and someone has loaded a mapped page table. > * >- * %rsi holds a physical pointer to real_mode_data. >- * > * We come here either from startup_64 (using physical addresses) > * or from trampoline.S (using virtual addresses). > * >@@ -197,13 +194,9 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL) > * hypervisor could lie about the C-bit position to perform a ROP > * attack on the guest by writing to the unencrypted stack and wait for > * the next RET instruction. >- * %rsi carries pointer to realmode data and is callee-clobbered. Save >- * and restore it. > */ >- pushq %rsi > movq %rax, %rdi > call sev_verify_cbit >- popq %rsi > > /* > * Switch to new page-table >@@ -294,9 +287,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL) > wrmsr > > /* Setup and Load IDT */ >- pushq %rsi > call early_setup_idt >- popq %rsi > > /* Check if nx is implemented */ > movl $0x80000001, %eax >@@ -332,9 +323,9 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL) > pushq $0 > popfq > >- /* rsi is pointer to real mode structure with interesting info. >+ /* R12 is pointer to real mode structure with interesting info. > pass it to C */ >- movq %rsi, %rdi >+ movq %r12, %rdi > > .Ljump_to_C_code: > /*
Would it not make more sense to write it into a memory variable and accessing that variable from the C code by name?
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