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

[Thomas Gleixner]

On Tue, 30 Sep 2014, Andy Lutomirski wrote:
> It would certainly be possible to clear NT and retry IRET if IRET
> fails with NT set.  This would have no overhead for anything relevant.
> That would be this alternative from my 0/2 email:
> 
>  - Don't filter NT on sysenter.  Instead, filter it on EFI entry
>    and modify the IRET code to retry without NT set if NT was set.
> 
> Thomas hpa, etc: any thoughts?

Filter it right away. That's solid and obvious. Anything else is just
complex and prone for future brown paperbag failures.

We get the context switch benefit from it, so there is some
compensation for the extra cycles.

Thanks,

	tglx
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