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

H. Peter Anvin
2014-10-01T15:22:36Z

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.

-hpa