git: d0117d06e9e3 - stable/13 - linux(4): Add a comment explaining registers at syscall entry point on amd64

The branch stable/13 has been updated by dchagin:

URL: https://cgit.FreeBSD.org/src/commit/?id=d0117d06e9e3588a902119b7a011c3613ef4545c

commit d0117d06e9e3588a902119b7a011c3613ef4545c
Author:     Dmitry Chagin <dchagin@FreeBSD.org>
AuthorDate: 2023-05-28 14:06:05 +0000
Commit:     Dmitry Chagin <dchagin@FreeBSD.org>
CommitDate: 2023-06-29 08:16:01 +0000

    linux(4): Add a comment explaining registers at syscall entry point on amd64
    
    Differential Revision:  https://reviews.freebsd.org/D40153
    MFC after:              1 month
    
    (cherry picked from commit a463dd810851fdae5efcb8f8205d742ebf221bc8)
---
 sys/amd64/linux/linux_sysvec.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/sys/amd64/linux/linux_sysvec.c b/sys/amd64/linux/linux_sysvec.c
index f43e7d3a02bc..3681dc881485 100644
--- a/sys/amd64/linux/linux_sysvec.c
+++ b/sys/amd64/linux/linux_sysvec.c
@@ -124,6 +124,23 @@ LINUX_VDSO_SYM_INTPTR(kern_timekeep_base);
 LINUX_VDSO_SYM_INTPTR(kern_tsc_selector);
 LINUX_VDSO_SYM_INTPTR(kern_cpu_selector);
 
+/*
+ * According to the Intel x86 ISA 64-bit syscall
+ * saves %rip to %rcx and rflags to %r11. Registers on syscall entry:
+ * %rax  system call number
+ * %rcx  return address
+ * %r11  saved rflags
+ * %rdi  arg1
+ * %rsi  arg2
+ * %rdx  arg3
+ * %r10  arg4
+ * %r8   arg5
+ * %r9   arg6
+ *
+ * Then FreeBSD fast_syscall() move registers:
+ * %rcx -> trapframe.tf_rip
+ * %r10 -> trapframe.tf_rcx
+ */
 static int
 linux_fetch_syscall_args(struct thread *td)
 {