Re: RFC: userspace exception fixups

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Mon, Nov 19, 2018 at 09:00:08AM -0800, Andy Lutomirski wrote:
> On Mon, Nov 19, 2018 at 8:02 AM Jarkko Sakkinen
> <jarkko.sakkinen@linux.intel.com> wrote:
> >
> > On Mon, Nov 19, 2018 at 07:29:36AM -0800, Andy Lutomirski wrote:
> > > 1. The kernel needs some way to know *when* to apply this fixup.
> > > Decoding the instruction stream and doing it to all exceptions that
> > > hit an ENCLU instruction seems like a poor design.
> >
> > I'm not sure why you would ever need to do any type of fixup as the idea
> > is to just return to AEP i.e. from chosen exceptions (EPCM, #UD) the AEP
> > would work the same way as for exceptions that the kernel can deal with
> > except filling the exception information to registers.
> 
> Sure, but how does the kernel know when to do that and when to send a
> signal?  I don't really like decoding the instruction stream to figure
> it out.

Hmm... why you have to decode instruction stream to find that out? Would
just depend on exception type (#GP with EPCM, #UD). Or are you saying
that kernel should need to SIGSEGV if there is in fact ENCLU so that
there is no infinite trap loop? Sorry, I'm a bit lost here that where
does this decoding requirement comes from in the first place. I
understand how it is used in Sean's proposal...

Anyway, this option can be probably discarded without further
consideration because apparently single stepping can cause #DB SS fault
if AEP handler is anything else than a single instruction.

For me it seems that by ruling out options, vDSO option is what is
left. I don't like it but at least it works...

/Jarkko