Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

H. Peter Anvin
2014-10-01T15:26:55Z

On 10/01/2014 08:22 AM, H. Peter Anvin wrote:
> On 09/30/2014 09:51 PM, Andy Lutomirski wrote:
>>
>> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
>> index 4299eb05023c..44d1dd371454 100644
>> --- a/arch/x86/ia32/ia32entry.S
>> +++ b/arch/x86/ia32/ia32entry.S
>> @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target)
>> 1: movl (%rbp),%ebp
>> _ASM_EXTABLE(1b,ia32_badarg)
>> ASM_CLAC
>> +
>> + /*
>> + * Sysenter doesn't filter flags, so we need to clear NT
>> + * ourselves. To save a few cycles, we can check whether
>> + * NT was set instead of doing an unconditional popfq.
>> + */
>> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */
>> + jz 1f
>> + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
>> + popfq_cfi
>> +1:
>> +
>
> I'm wondering if it would be easier to just remove ASM_CLAC and do this
> unconditionally. On SMAP-enabled hardware then that gives us back some
> of the cycles, may make the branch unnecessary.
>

Heck, we can drop the CLD and the STI as well (with some tweaking in
ia32_badarg.)

-hpa