Messages in this thread | | | From | Brian Gerst <> | Date | Sat, 1 Apr 2023 01:06:51 -0400 | Subject | Re: [PATCH] x86/boot: Clean up handling of boot_params pointer |
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On Fri, Mar 31, 2023 at 9:00 PM H. Peter Anvin <hpa@zytor.com> wrote: > > 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?
I think ideally we'd want to copy the real mode data as early as possible. However I don't know how that would interact with memory encryption. By reading the code, I think it would work, but I don't have the hardware to test it.
-- Brian Gerst
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