Re: [PATCHv2 4/4] x86_64, entry: Create IRET-compatible stack frame at syscall entry

[Alexander van Heukelum <heukelum@fastmail.fm>]

On Sun, Jan 18, 2015, at 17:38, Andy Lutomirski wrote:
> On Sun, Jan 18, 2015 at 3:45 AM, Alexander van Heukelum
> <heukelum@fastmail.fm> wrote:
> > Create an IRET-compatible top of stack at syscall entry and use this
> > information to return to user mode in the sysret path. This removes
> > the need for the FIXUP_TOP_OF_STACK and RESTORE_TOP_OF_STACK macros.
> 
> Since I have limited bandwidth, I'd like to tackle these one at a time.
> 
> I like the idea of this patch, but it has some issues.
> 
> First, it needs to be benchmarked.  The syscall fast path entry code
> is *very* hot in some workloads, and it needs to be fast.

Yeah. I didn't even try to benchmark it. What do you propose to use in this case?

> Second, I think you're really making three changes here.
> 
> a) You're putting rsp where it belongs -- it's in pt_regs instead of
> being magically shoved into a combination of per-cpu variables and
> extra arch state (thread->usersp).  This ideally consists of (AFAICS)
> two tiny asm changes: one extra mov (most likely cache-hot) on entry
> and a change of where you're reading from when you reload rsp on exit.
> The former change could easily add a cycle (or zero cycles, or lots of
> cycles -- hardware can be complicated, and I have no idea how well
> store forwarding works on gs-relative accesses).  The latter change is
> probably a speedup -- we'd be reading from pt_regs (almost certainly
> hot or at least easily detected by the hardware prefetcher) instead of
> a random percpu variable on exit.
> 
> *However*, this change enables the removal of all the usersp crap when
> context switching, and all of the old_rsp references need to be
> audited, and (having added yet another of them a week or two ago) I
> know that you missed at least one and probably three or four :)  Also,
> removing the usersp crap could easily speed up context switches by a
> cache line or so.

Yes. I missed that part. I'll  look into it. But nothing seems to blow up :)

> Can you do that and split out just the old_rsp, usersp, and rsp part
> as its own patch?
> 
> b) You're putting the saved flags into the EFLAGS pt_regs slot, which
> seems to me to be an unambiguous win -- it removes two instructions
> from RESTORE_TOP_OF_STACK, and it adds nothing whatsoever (except to
> the extent that you continue to initialize R11 on entry instead of in
> FIXUP_TOP_OF_STACK).
> 
> (a) and (b) alone should be enough to eliminate RESTORE_TOP_OF_STACK.
> 
> c) You're initializing the rest of the "top of stack" (cs, ss, and
> rcx) unconditionally.  This is simpler, but I'm not sure it's
> worthwhile -- we still lazily save the caller-saved regs, and
> FIXUP_TOP_OF_STACK fits right in.  It also may have a performance
> impact.
> 
> I think that (a) and (b) are clear wins (a is a really nice cleanup
> and I bet it's a speedup, too, and b seems to be better in all
> respects).  (c) is much less clearly a win to me.
> 
> Would you be willing to send split-out patches along with benchmarks?

Timing will depend highly on amounts of spare time, but I will give it
a shot.

Thanks for your valuable input!

Greetings,
    Alexander 

> --Andy