[Qemu-devel] [PATCH v2] target-mips: Fix exceptions while UX=0

[Qemu-devel] [PATCH v2] target-mips: Fix exceptions while UX=0

[James Hogan]

Commit 01f728857941 ("target-mips: Status.UX/SX/KX enable 32-bit address
wrapping") added a new hflag MIPS_HFLAG_AWRAP, which indicates that
64-bit addressing is disallowed in the current mode, so hflag users
don't need to worry about the complexities of working that out, for
example checking both MIPS_HFLAG_KSU and MIPS_HFLAG_UX.

However when exceptions are taken outside of exception level,
mips_cpu_do_interrupt() manipulates the env->hflags directly rather than
using compute_hflags() to update them, and this code wasn't updated
accordingly. As a result, when UX is cleared, MIPS_HFLAG_AWRAP is set,
but it doesn't get cleared on entry back into kernel mode due to an
exception. Kernel mode then cannot access the 64-bit segments resulting
in a nested exception loop. The same applies to errors and debug
exceptions.

Fix by updating mips_cpu_do_interrupt() to clear the MIPS_HFLAG_WRAP
flag when necessary, according to compute_hflags().

Fixes: 01f728857941 ("target-mips: Status.UX/SX/KX enable 32-bit...")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
---
Changes in v2:
- Add cases for debug exceptions and errors (Leon).
---
 target-mips/helper.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/target-mips/helper.c b/target-mips/helper.c
index b3fe816fecf8..118072a9e743 100644
--- a/target-mips/helper.c
+++ b/target-mips/helper.c
@@ -524,6 +524,10 @@ void mips_cpu_do_interrupt(CPUState *cs)
  enter_debug_mode:
         if (env->insn_flags & ISA_MIPS3) {
             env->hflags |= MIPS_HFLAG_64;
+            if (!(env->insn_flags & ISA_MIPS64R6) ||
+                env->CP0_Status & (1 << CP0St_KX)) {
+                env->hflags &= ~MIPS_HFLAG_AWRAP;
+            }
         }
         env->hflags |= MIPS_HFLAG_DM | MIPS_HFLAG_CP0;
         env->hflags &= ~(MIPS_HFLAG_KSU);
@@ -548,6 +552,10 @@ void mips_cpu_do_interrupt(CPUState *cs)
         env->CP0_Status |= (1 << CP0St_ERL) | (1 << CP0St_BEV);
         if (env->insn_flags & ISA_MIPS3) {
             env->hflags |= MIPS_HFLAG_64;
+            if (!(env->insn_flags & ISA_MIPS64R6) ||
+                env->CP0_Status & (1 << CP0St_KX)) {
+                env->hflags &= ~MIPS_HFLAG_AWRAP;
+            }
         }
         env->hflags |= MIPS_HFLAG_CP0;
         env->hflags &= ~(MIPS_HFLAG_KSU);
@@ -725,6 +733,10 @@ void mips_cpu_do_interrupt(CPUState *cs)
             env->CP0_Status |= (1 << CP0St_EXL);
             if (env->insn_flags & ISA_MIPS3) {
                 env->hflags |= MIPS_HFLAG_64;
+                if (!(env->insn_flags & ISA_MIPS64R6) ||
+                    env->CP0_Status & (1 << CP0St_KX)) {
+                    env->hflags &= ~MIPS_HFLAG_AWRAP;
+                }
             }
             env->hflags |= MIPS_HFLAG_CP0;
             env->hflags &= ~(MIPS_HFLAG_KSU);
-- 
2.4.10

Re: [Qemu-devel] [PATCH v2] target-mips: Fix exceptions while UX=0

[Leon Alrae]

On 17/11/15 17:13, James Hogan wrote:
> Commit 01f728857941 ("target-mips: Status.UX/SX/KX enable 32-bit address
> wrapping") added a new hflag MIPS_HFLAG_AWRAP, which indicates that
> 64-bit addressing is disallowed in the current mode, so hflag users
> don't need to worry about the complexities of working that out, for
> example checking both MIPS_HFLAG_KSU and MIPS_HFLAG_UX.
> 
> However when exceptions are taken outside of exception level,
> mips_cpu_do_interrupt() manipulates the env->hflags directly rather than
> using compute_hflags() to update them, and this code wasn't updated
> accordingly. As a result, when UX is cleared, MIPS_HFLAG_AWRAP is set,
> but it doesn't get cleared on entry back into kernel mode due to an
> exception. Kernel mode then cannot access the 64-bit segments resulting
> in a nested exception loop. The same applies to errors and debug
> exceptions.
> 
> Fix by updating mips_cpu_do_interrupt() to clear the MIPS_HFLAG_WRAP
> flag when necessary, according to compute_hflags().
> 
> Fixes: 01f728857941 ("target-mips: Status.UX/SX/KX enable 32-bit...")
> Signed-off-by: James Hogan <james.hogan@imgtec.com>
> Cc: Leon Alrae <leon.alrae@imgtec.com>
> Cc: Aurelien Jarno <aurelien@aurel32.net>
> ---
> Changes in v2:
> - Add cases for debug exceptions and errors (Leon).
> ---
>  target-mips/helper.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)

Applied to target-mips queue, thanks.

Leon