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

[Thomas Gleixner]

On Tue, 30 Sep 2014, Andy Lutomirski wrote:

> On Tue, Sep 30, 2014 at 3:27 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > 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.
> 
> Yeah, agreed.  That's exactly what these patches do, although, if you
> put them in -tip and want to keep the stable CC, it's probably worth
> fixing the address (oops).

Even more oops as you failed to update it in your reply again ....
 
> > We get the context switch benefit from it, so there is some
> > compensation for the extra cycles.
> 
> If we ever want those cycles back, I bet that the compat sysenter path
> could be trimmed down a lot.  For example, I think that all of the
> zero-extension stuff is unnecessary now that we have the magic syscall
> wrappers for all (?) syscalls.

Emphasis on "(?)". So yes, once we verified that ....

Thanks,

	tglx
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