On Sun, Jan 21, 2018 at 07:29:43PM +0100, KarimAllah Ahmed wrote:
> Because static_cpu_has is an indirect branch which will cause speculation
> and
> we have to avoid that.
How so?
The JMP_NOSPEC macro protects against JMP <reg> jumps but the
static_cpu_has() macros all add JMPs with an immediate offset from the
next instruction and I wouldn't call them indirect JMPs as there are no
registers to speculate on there.
IOW, before alternatives, the patch site of static_cpu_has() looks like this:
# 151 "./arch/x86/include/asm/cpufeature.h" 1
1: jmp 6f
and that 6f label is:
6:
testb $1,boot_cpu_data+50(%rip) #, MEM[(const char *)&boot_cpu_data +
50B]
jnz .L707 #
jmp .L706 #
i.e., we basically do if (boot_cpu_has(..)).
If the feature is not present, same patch site turns into:
4: jmp .L706 #
5:
after patching. Which is a label after the whole thing. That is not an
indrect jump through a register either.
If the feature is present, the patch site becomes:
NOP - added by the patching
# ./arch/x86/include/asm/msr.h:105: asm volatile("1: wrmsr\n"
.loc 18 105 0
movl $73, %ecx #, tmp138
movl $1, %eax #, tmp139
xorl %edx, %edx # tmp140
#APP
# 105 "./arch/x86/include/asm/msr.h" 1
1: wrmsr
2:
so execution runs directly into the MSR write and the JMP is gone.
So I don't see indirect branches anywhere...
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
