On Mon, May 23, 2016 at 4:02 PM, Jiri Kosina [off-list ref] wrote:

On Fri, 20 May 2016, Andy Lutomirski wrote:

quoted

I think it would be negligible, at least for interrupts, since
interrupts are already extremely expensive.  But I don't love adding
assembly code that makes them even slower.  The real thing I dislike
about this approach is that it's not a normal stack frame, so you need
code in the unwinder to unwind through it correctly, which makes me
think that you're not saving much complexity by adding the pushes.

I fail to see what is so special about the stack frame; it's in fact
pretty normal.

It has added semantic value for "those who know", but the others will
(pretty much correctly) consider it to be a stackframe from a function
call, and be done with it.

What am I missing?

In Josh's code, the stack looks like:

...
interrupted frame
pt_regs
pointer to pt_regs
address of pt_regs dummy function
rbp
handler frame

A naive unwinder won't unwind this correctly, as there's no link back
to the interrupted frame's RIP.  If the layout changed to:


...
interrupted frame
pt_regs
interrupted RIP
rbp
handler frame

then I think it would unwind correctly, but the pt_regs would be
invisible, which is IMO a bit unfortunate.  It could also be (I
think):


...
interrupted frame
pt_regs
interrupted rbp
interrupted RIP
pointer to pt_regs
address of pt_regs dummy function
pointer to "interrupted RIP" stack slot
handler frame

but now this is *five* pushes for the dummy frame, which I think is
getting a bit out of hand.

--Andy