在 2018/2/14 17:58, Jan Beulich 写道:
On 14.02.18 at 10:25, <zhenzhong.d...@oracle.com> wrote:
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -269,28 +269,29 @@
* This is logical merge of DO_OVERWRITE_RSB and DO_SPEC_CTRL_ENTRY
* maybexen=1, but with conditionals rather than alternatives.
*/
- movzbl STACK_CPUINFO_FIELD(bti_ist_info)(%r14), %eax
+ movzbl STACK_CPUINFO_FIELD(bti_ist_info)(%r14), %edx
- testb $BTI_IST_RSB, %al
+ testb $BTI_IST_RSB, %dl
jz .L\@_skip_rsb
DO_OVERWRITE_RSB
.L\@_skip_rsb:
- testb $BTI_IST_WRMSR, %al
+ testb $BTI_IST_WRMSR, %dl
jz .L\@_skip_wrmsr
+ mov %edx, %eax
xor %edx, %edx
testb $3, UREGS_cs(%rsp)
setz %dl
and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14)
-.L\@_entry_from_xen:
/*
* Load Xen's intended value. SPEC_CTRL_IBRS vs 0 is encoded in the
* bottom bit of bti_ist_info, via a deliberate alias with BTI_IST_IBRS.
*/
+ xor %edx, %edx
mov $MSR_SPEC_CTRL, %ecx
and $BTI_IST_IBRS, %eax
wrmsr
While indeed you add one less instruction, you don't shrink overall
code size compared to v2. I also prefer v2 because of being more
explicit about the register needing to be preserved across
DO_OVERWRITE_RSB.
Then Ok, in fact my inital thought is to avoid unnecessory mov
instructions around DO_OVERWRITE_RSB in the 'jmp _skip_wrmsr' case, so
tried to remove them.
--
thanks
zduan
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