Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 30 Sep 2014 22:24:35 -0700 | Subject | Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace |
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On Tue, Sep 30, 2014 at 10:09 PM, Sebastian Lackner <sebastian@fds-team.de> wrote: >> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */ >> + jz 1f >> + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED) >> + popfq_cfi >> +1: >> + > > Do you think it makes sense to change the order here, so that no jump happens if > NT is not set (which happens a bit more often, than the other way round)? Just a > guess though, haven't measured if pipeline effects have such a big influence in this > case. ;) >
It should be immeasurable in a tight loop, since it will predict correctly almost every time. And, unless cfi state works across .pushsection (does it?), getting the cfi annotations right will be more complicated.
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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