Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs
From: Denys Vlasenko
Date: Sat Mar 12 2016 - 12:53:20 EST
On 03/12/2016 04:38 PM, Ingo Molnar wrote:
> * 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)
test_syscall_vdso32 checks vdso syscall (if available)
and direct int80 syscall.
Booting two times, with different qemu flags:
qemu-system-x86_64 -cpu Opteron_G4
qemu-system-x86_64 -cpu SandyBridge
makes kernel choose either SYSCALL or SYSENTER vdso.
So it's all covered.