System calls using the sysret exit path (or its VGCF_in_syscall IRET
equivalent on Xen) always go into user mode, hence the conditionals
in HYPERVISOR_IRET can be omitted entirely.
Further, there's no need to spill %rcx to the stack upon syscall entry
(neither does native code): No argument gets passed there, and due to
the use of the syscall instruction the register's contents don't need
to be preserved either.
Finally, rather than restoring %rcx and %r11 (not preserved by syscall/
sysret anyway) from the stack frame, we can as well simply clear them
out (to avoid leaking kernel data), thus reducing the necessary memory
(cache) bandwidth.
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
--- a/arch/x86_64/kernel/entry-xen.S
+++ b/arch/x86_64/kernel/entry-xen.S
@@ -153,6 +153,7 @@ NMI_MASK = 0x80000000
* with rax, r11, and rcx being taken care of in the hypercall stub.
*/
.macro HYPERVISOR_IRET flag
+ .if \flag == 0 # return from syscall always uses the hypercall
testb $3,1*8(%rsp)
jnz 2f
testl $NMI_MASK,2*8(%rsp)
@@ -165,6 +166,7 @@ NMI_MASK = 0x80000000
orl $3,1*8(%rsp)
orl $3,4*8(%rsp)
1: iretq
+ .endif
2: /* Slow iret via hypervisor. */
andl $~NMI_MASK, 2*8(%rsp)
@@ -248,7 +250,7 @@ END(ret_from_fork)
ENTRY(system_call)
_frame (RIP-0x10)
- SAVE_ARGS -8,0
+ SAVE_ARGS -8,1
movq %rax,ORIG_RAX-ARGOFFSET(%rsp)
GET_THREAD_INFO(%rcx)
testl
$(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),threadinfo_flags(%rcx)
@@ -279,8 +281,9 @@ sysret_check:
* sysretq will re-enable interrupts:
*/
TRACE_IRQS_ON
- XEN_UNBLOCK_EVENTS(%rsi)
- RESTORE_ARGS 0,8,0
+ RESTORE_ARGS 0,8,1,1
+ xor %ecx,%ecx
+ xor %r11,%r11
HYPERVISOR_IRET VGCF_IN_SYSCALL
/* Handle reschedules */
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