Re: [Xen-devel] [PATCH 2/2] x86/ldt: allow to disable modify_ldt at runtime

[Willy Tarreau <w@xxxxxx>]

On Mon, Aug 03, 2015 at 12:06:12PM -0700, Andy Lutomirski wrote:
> On Mon, Aug 3, 2015 at 12:01 PM, Willy Tarreau <w@xxxxxx> wrote:
(...)
> > I feel like it's probably part of a larger project then. Do you think
> > we should step back and only support 0/1 for now ? I also have the
> > patch available.
> 
> Sounds good to me.

OK I'll send the other one instead once I unpack my PC.

> > Well the good thing is that SYSRET reused the LOADALL opcode so at
> > least this one cannot be screwed on 64-bit :-) It would have helped us
> > to emulate IRET though.
> 
> You sure?  I'm reasonably confident that Athlon 64 and newer support
> SYSRET in legacy and long mode.  Of course, I think that SYSCALL is
> totally worthless in legacy mode (SYSCALL_MASK isn't available, so I
> suspect that the lack of sensible TF handling would be a
> show-stopper).

I meant loadall cannot be screwed since it was replaced.

> >> P.P.S. You know what would be *way* better than allowing IRET to
> >> fault?  Just allow IRET to continue executing the next instruction on
> >> failure (I'm talking about #GP, #NP, and #SS here, not page faults).
> >>
> >> P.P.P.S.  Who thought that IRET faults unmasking NMIs made any sense
> >> whatsoever when NMIs run on an IST stack?  Seriously, people?
> >
> > A dedicated flag "don't clear NMI yet" would have been nice in EFLAGS
> > so that the software stack could set it in fault handlers. It would be
> > one-shot and always cleared by IRET. That would have been very handy.
> >
> 
> How about a dedicated "NMI masked" flag in EFLAGS?  That would be
> straightforward and dead simple to handle.

Sounds like an oxymoron. But such a flag should be atomically manipulated
so that you don't re-arm queued NMIs before calling iret. With two flags,
a read-only one for "NMI masked" and a modifiable one "keep NMI masked",
you can provide an atomic behaviour where you have this latch executed
on iret :

     NMI_MASKED &= KEEP_NMI_MASKED;
     KEEP_NMI_MASKED = 0;

But anyway we're discussing in the void, this CPU doesn't exist so unless
intel/AMD designers want to improve their design (and start by talking
together to reach the exact same behavior), we'll never see anything like
this :-/

Willy


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