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

[Andy Lutomirski]

On Sun, Jan 18, 2015 at 3:45 AM, Alexander van Heukelum
<heukelum@xxxxxxxxxxx> 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.

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.

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?

--Andy
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