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Unconditionally restore ar.k7 (memory stack) and ar.k6 (register stack)

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when returning from an interrupt. Both registers are used on interrupt
to switch to the right kernel stack, but other than that they are not
used. This means we only have to make sure they contain proper values
while in user mode. As such, we conditionally restored these registers
based on whether we returned to userland or not. A nice property of
conditionally restoring ar.k6 and ar.k7 is that it introduces two
invariants: ar.k6 always points to the bottom of the kernel stack and
ar.k7 always points to the top of the kernel stack (immediately below
the PCB we have there).

However, the EPC syscall path introduces an irregularity: there's no
"thin red line" between user and kernel. There's a grey area that's a
couple of instructions wide. Any interruption in that grey area is
bound to see an inconsistent state. One such state is that we're in
kernel space for all practical purposes, but we still need to have
ar.k6 and ar.k7 restored as if we're in userland.

Thus: restore ar.k6 and ar.k7 unconditionally at the cost of losing
a valuable invariant. Both registers now hold the extend of the
usable portion of the kernel stack at any interrupt nesting, which
when in userland mean the bottom and the top of the kstack.
This commit is contained in:
marcel 2003-05-24 20:51:55 +00:00
parent bc1f5bcccb
commit c4e56d4da9
2 changed files with 4 additions and 4 deletions
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4
sys/ia64/ia64/exception.S
View File

@ -522,7 +522,7 @@ ENTRY(exception_restore, 0)
// the backing store.
{ .mmi
mov ar.rsc=r31 // setup for loadrs
(p15) mov ar.k7=r16
mov ar.k7=r16
(p15) mov r13=r29
;;
}
@ -542,7 +542,7 @@ exception_restore_restart:
;;
}
{ .mmb
(p15) mov ar.k6=r31
mov ar.k6=r31
mov ar.rnat=r21
nop 0
;;

4
sys/ia64/ia64/exception.s
View File

@ -522,7 +522,7 @@ ENTRY(exception_restore, 0)
// the backing store.
{ .mmi
mov ar.rsc=r31 // setup for loadrs
(p15) mov ar.k7=r16
mov ar.k7=r16
(p15) mov r13=r29
;;
}
@ -542,7 +542,7 @@ exception_restore_restart:
;;
}
{ .mmb
(p15) mov ar.k6=r31
mov ar.k6=r31
mov ar.rnat=r21
nop 0
;;