[Linux-ia64] sigaltstack and ar.bspstore

[Linux-ia64] sigaltstack and ar.bspstore

[Matt Chapman]

Currently when using an alternate stack for signal handling, the
trampoline code switches to the new register stack without saving
bspstore.  When returning to the original register stack, it calculates
the bspstore as:

  bspstore0 = rse_skip_regs(bsp0, -rse_num_regs(bsp1 - (loadrs >> 19), bsp1));
  where bsp0 is sc->sc_ar_bsp and bsp1 is the current bsp

Presumably the result should be the same as the original bspstore.
However, it isn't in the case when the original bspstore is pointing to
slot 63 (the RNAT word).  The new bspstore comes out pointing to slot 0
of the next group, and the RNAT bits never get written.  This happens
more often then you'd expect, since it seems that current
implementations make the optimisation of writing the RNAT word together
with the first word of the new group instead of the last word of the old
group (sensible because it potentially saves a write).

I'd propose that the trampoline code should save bspstore in the
sigcontext structure and restore it from there rather than recalculating
it.  I can send a patch, but first is there any reason why it shouldn't
be done that way?

Matt

Re: [Linux-ia64] sigaltstack and ar.bspstore

[David Mosberger]

>>>>> On Sat, 31 May 2003 00:18:39 +1000, Matt Chapman <matthewc@cse.unsw.edu.au> said:

  Matt> Presumably the result should be the same as the original
  Matt> bspstore.  However, it isn't in the case when the original
  Matt> bspstore is pointing to slot 63 (the RNAT word).  The new
  Matt> bspstore comes out pointing to slot 0 of the next group, and
  Matt> the RNAT bits never get written.  This happens more often then
  Matt> you'd expect, since it seems that current implementations make
  Matt> the optimisation of writing the RNAT word together with the
  Matt> first word of the new group instead of the last word of the
  Matt> old group (sensible because it potentially saves a write).

Yes, I noticed the same in libunwind. ;-/

  Matt> I'd propose that the trampoline code should save bspstore in
  Matt> the sigcontext structure and restore it from there rather than
  Matt> recalculating it.  I can send a patch, but first is there any
  Matt> reason why it shouldn't be done that way?

My preference would be to fix it in a way that preserves binary
compatibility for the sigcontext structure.

	--david

Re: [Linux-ia64] sigaltstack and ar.bspstore

[Matt Chapman]

On Fri, May 30, 2003 at 09:32:03AM -0700, David Mosberger wrote:
> 
>   Matt> I'd propose that the trampoline code should save bspstore in
>   Matt> the sigcontext structure and restore it from there rather than
>   Matt> recalculating it.  I can send a patch, but first is there any
>   Matt> reason why it shouldn't be done that way?
> 
> My preference would be to fix it in a way that preserves binary
> compatibility for the sigcontext structure.

I wasn't suggesting changing the layout, I was suggesting using one of
the reserved slots.  Unless you're referring to software that changes
sc_ar_bsp...

I don't think there's currently enough information available upon return
from the signal handler to decide whether to point bspstore at slot 63
or 0.  It wouldn't be safe to always go for slot 63 because it might be
beyond the start of the RBS.

Matt

Re: [Linux-ia64] sigaltstack and ar.bspstore

[David Mosberger]

>>>>> On Sat, 31 May 2003 11:31:18 +1000, Matt Chapman <matthewc@cse.unsw.edu.au> said:

  Matt> I wasn't suggesting changing the layout, I was suggesting
  Matt> using one of the reserved slots.  Unless you're referring to
  Matt> software that changes sc_ar_bsp...

Yes, that's what I was referring to.

  Matt> I don't think there's currently enough information available
  Matt> upon return from the signal handler to decide whether to point
  Matt> bspstore at slot 63 or 0.  It wouldn't be safe to always go
  Matt> for slot 63 because it might be beyond the start of the RBS.

I'll have to think about this some more.  Especially to make sure that
there aren't any other places were we (I) made the assumption that
bspstore never points to an RNaT slot.  But it will take me a while
before I get to it (in case I forget or you come up with a great
solution, please ping me... ;-).

	--david

Re: [Linux-ia64] sigaltstack and ar.bspstore

[David Mosberger]

>>>>> On Sat, 31 May 2003 00:18:39 +1000, Matt Chapman <matthewc@cse.unsw.edu.au> said:

  Matt> Currently when using an alternate stack for signal handling,
  Matt> the trampoline code switches to the new register stack without
  Matt> saving bspstore.  When returning to the original register
  Matt> stack, it calculates the bspstore as:

  Matt>   bspstore0 = rse_skip_regs(bsp0, -rse_num_regs(bsp1 - (loadrs
  Matt> >> 19), bsp1)); where bsp0 is sc->sc_ar_bsp and bsp1 is the
  Matt> current bsp

  Matt> Presumably the result should be the same as the original
  Matt> bspstore.  However, it isn't in the case when the original
  Matt> bspstore is pointing to slot 63 (the RNAT word).  The new
  Matt> bspstore comes out pointing to slot 0 of the next group, and
  Matt> the RNAT bits never get written.

Yes, this is a problem.  Can you try the attached patch?  Rather than
storing an additional sc_bspstore member, the backing-store switching
code checks whether bspstore points to an RNaT slot and, if so, stores
the RNaT manually.  This effectively "normalizes" the backing-store
such that ar.bspstore never points to an RNaT slot.

Note that binary compatibility is the main reason for this solution.
Perhaps some day there will be an opportunity to redo the sigcontext
structure and at that point we could store the bspstore/loadrs pair
(as we do on kernel entry) instead of the bsp/loadrs pair that we
currently store in sigcontext.

	--david

=== arch/ia64/kernel/gate.S 1.17 vs edited ==--- 1.17/arch/ia64/kernel/gate.S	Tue Jun 17 23:50:16 2003
+++ edited/arch/ia64/kernel/gate.S	Fri Jun 20 22:26:34 2003
@@ -199,7 +199,7 @@
 	cmp.ne p8,p0=r15,r0		// do we need to switch the rbs?
 	mov.m r9=ar.bsp			// fetch ar.bsp
 	.spillsp.p p8, ar.rnat, RNAT_OFF+SIGCONTEXT_OFF
-(p8)	br.cond.spnt setup_rbs		// yup -> (clobbers r14, r15, and r16)
+(p8)	br.cond.spnt setup_rbs		// yup -> (clobbers p8, r14-r16, and r18-r20)
 back_from_setup_rbs:
 	alloc r8=ar.pfs,0,0,3,0
 	ld8 out0=[base0],16		// load arg0 (signum)
@@ -268,26 +268,30 @@
 setup_rbs:
 	mov ar.rsc=0				// put RSE into enforced lazy mode
 	;;
-	.save ar.rnat, r16
-	mov r16=ar.rnat				// save RNaT before switching backing store area
+	.save ar.rnat, r19
+	mov r19=ar.rnat				// save RNaT before switching backing store area
 	adds r14=(RNAT_OFF+SIGCONTEXT_OFF),sp
 
+	mov r18=ar.bspstore
 	mov ar.bspstore=r15			// switch over to new register backing store area
 	;;
+
 	.spillsp ar.rnat, RNAT_OFF+SIGCONTEXT_OFF
-	st8 [r14]=r16				// save sc_ar_rnat
+	st8 [r14]=r19				// save sc_ar_rnat
 	.body
-	adds r14=(LOADRS_OFF+SIGCONTEXT_OFF),sp
-
 	mov.m r16=ar.bsp			// sc_loadrs <- (new bsp - new bspstore) << 16
+	adds r14=(LOADRS_OFF+SIGCONTEXT_OFF),sp
 	;;
 	invala
 	sub r15=r16,r15
+	extr.u r20=r18,3,6
 	;;
+	mov ar.rsc=0xf				// set RSE into eager mode, pl 3
+	cmp.eq p8,p0c,r20
 	shl r15=r15,16
 	;;
 	st8 [r14]=r15				// save sc_loadrs
-	mov ar.rsc=0xf				// set RSE into eager mode, pl 3
+(p8)	st8 [r18]=r19		// if bspstore points at RNaT slot, store RNaT there now
 	.restore sp				// pop .prologue
 	br.cond.sptk back_from_setup_rbs
 

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Re: [Linux-ia64] sigaltstack and ar.bspstore

[Matt Chapman]

On Fri, Jun 20, 2003 at 11:05:30PM -0700, David Mosberger wrote:
> >>>>> On Sat, 31 May 2003 00:18:39 +1000, Matt Chapman <matthewc@cse.unsw.edu.au> said:
> 
>   Matt> Currently when using an alternate stack for signal handling,
>   Matt> the trampoline code switches to the new register stack without
>   Matt> saving bspstore.  When returning to the original register
>   Matt> stack, it calculates the bspstore as:
> 
>   Matt>   bspstore0 = rse_skip_regs(bsp0, -rse_num_regs(bsp1 - (loadrs
>   Matt> >> 19), bsp1)); where bsp0 is sc->sc_ar_bsp and bsp1 is the
>   Matt> current bsp
> 
>   Matt> Presumably the result should be the same as the original
>   Matt> bspstore.  However, it isn't in the case when the original
>   Matt> bspstore is pointing to slot 63 (the RNAT word).  The new
>   Matt> bspstore comes out pointing to slot 0 of the next group, and
>   Matt> the RNAT bits never get written.
> 
> Yes, this is a problem.  Can you try the attached patch?  Rather than
> storing an additional sc_bspstore member, the backing-store switching
> code checks whether bspstore points to an RNaT slot and, if so, stores
> the RNaT manually.  This effectively "normalizes" the backing-store
> such that ar.bspstore never points to an RNaT slot.

Ah, that's a good way of doing it (given the constraints of backwards
compatibility).  I've tested your patch and it works well for me.

Thanks!

Matt


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Re: [Linux-ia64] sigaltstack and ar.bspstore

[David Mosberger]

>>>>> On Sat, 21 Jun 2003 16:58:59 +1000, Matt Chapman <matthewc@cse.unsw.edu.au> said:

  Matt> I've tested your patch and it works well for me.

Great!  Thanks for testing it.

	--david

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