[RFC 09/23] x86_64: Store userspace rsp in system_call fastpath

[Jiri Olsa]

hi,
I'd need help with this change.. basically it works, but I guess
I could alter the FIXUP_TOP_OF_STACK macro as well, so the rsp is
not initialized twice in slowpath.

But it seems quite complex.. so not really sure at the moment ;)

ideas?

thanks,
jirka

---
Storing the userspace rsp into the pt_regs struct for the
system_call fastpath (syscall instruction handler).

Following part of the pt_regs is allocated on stack:
(via KERNEL_STACK_OFFSET)

    unsigned long ip;
    unsigned long cs;
    unsigned long flags;
    unsigned long sp;
    unsigned long ss;

but only ip is actually saved for fastpath.

For perf post unwind we need at least ip and sp to be able to
start the unwind, so storing the old_rsp value to the sp.

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Roland McGrath <roland@xxxxxxxxxxxxx>
---
 arch/x86/kernel/entry_64.S |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 111f6bb..0444917 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -516,6 +516,11 @@ GLOBAL(system_call_after_swapgs)
 	SAVE_ARGS 8,0
 	movq  %rax,ORIG_RAX-ARGOFFSET(%rsp)
 	movq  %rcx,RIP-ARGOFFSET(%rsp)
+#ifdef CONFIG_PERF_EVENTS
+	/* We need rsp in fast path for perf post unwind. */
+	movq  PER_CPU_VAR(old_rsp), %rcx
+	movq  %rcx,RSP-ARGOFFSET(%rsp)
+#endif
 	CFI_REL_OFFSET rip,RIP-ARGOFFSET
 	testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
 	jnz tracesys
-- 
1.7.7.6

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