* Andy Lutomirski <l...@kernel.org> 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: sta...@vger.kernel.org
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.
Thanks,
Ingo
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