Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs
From: Ingo Molnar
Date: Sat Mar 12 2016 - 10:38:23 EST
* Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> Use of a temporary R8 register here seems to be unnecessary.
> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
> "push $0" is two-byte too. It seems just using the latter would be
> no worse.
> Thus, code had an unnecessary "xorq %r8,%r8" insn.
> It probably costs nothing in execution time here since we are probably
> limited by store bandwidth at this point, but still.
> Run-tested under QEMU: 32-bit calls still work:
> / # ./test_syscall_vdso32
Did you manage to test all 3 compat variants:
> @@ -72,24 +72,23 @@ ENTRY(entry_SYSENTER_compat)
> @@ -205,17 +204,16 @@ ENTRY(entry_SYSCALL_compat)
> @@ -316,11 +314,10 @@ ENTRY(entry_INT80_compat)