Re: [RFC] x86, ia32entry: Use sysretl to return from sysenter

[Andy Lutomirski]

On Mar 28, 2015 1:35 AM, "Ingo Molnar" <mingo@xxxxxxxxxx> wrote:
>
>
> * Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> > Sysexit is scary on 64-bit kernels -- sysexit must be invoked with
> > usergs and IRQs on.  That means that we rely on sti to correctly
> > mask interrupts for one instruction.  This is okay by itself, but
> > the semantics with respect to NMIs are unclear.
>
> At least judging by profiling output I think NMIs observe the STI
> window of one instruction non-execution as well. (But I'm not 100%
> sure.)
>
> > Avoid the whole issue by using sysretl instead.  For background,
> > Intel CPUs don't allow syscall from compat mode, but they do allow
> > sysret back to compat mode.  Go figure.
> >
> > Oddly this seems to be 30 cycles or so faster.  Avoiding popfq and
> > sti will account for under half of that, I think, so my best guess
> > is that Intel just optimizes sysret much better than sysexit.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
>
> I like it, but no way is this automatic -stable material ... if proven
> upstream we can forward it as a fix for SYSEXIT fragility, but not
> automatically, IMHO.

Agreed.  I wish we had a Stable-after-a-long-soak tag.

--Andy
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