[PATCH 0/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[PATCH 0/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[Ilya Leoshkevich]

Hi,

I noticed that certain accesses to lowcore incorrectly trigger
protection exceptions. I tracked it down to store_helper_unaligned()
calling tlb_fill() with ranges like [0, 2000).

Patch 1 fixes the issue, patch 2 adds a new MMIO device that enables
writing system tests for s390x, patch 3 adds a system test for this
issue.

Best regards,
Ilya

Ilya Leoshkevich (3):
  accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore
  hw/misc: Add mmio-debug-exit device
  tests/tcg/s390x: Test unaligned accesses to lowcore

 accel/tcg/cputlb.c                      |  8 ++-
 hw/misc/Kconfig                         |  3 +
 hw/misc/debugexit_mmio.c                | 80 +++++++++++++++++++++++++
 hw/misc/meson.build                     |  1 +
 hw/s390x/Kconfig                        |  1 +
 tests/tcg/s390x/Makefile.softmmu-target |  9 +++
 tests/tcg/s390x/unaligned-lowcore.S     | 24 ++++++++
 7 files changed, 123 insertions(+), 3 deletions(-)
 create mode 100644 hw/misc/debugexit_mmio.c
 create mode 100644 tests/tcg/s390x/Makefile.softmmu-target
 create mode 100644 tests/tcg/s390x/unaligned-lowcore.S

-- 
2.35.3

[PATCH 1/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[Ilya Leoshkevich]

If low-address-protection is active, unaligned stores to non-protected
parts of lowcore lead to protection exceptions. The reason is that in
such cases tlb_fill() call in store_helper_unaligned() covers
[0, addr + size) range, which contains the protected portion of
lowcore. This range is too large.

The most straightforward fix would be to make sure we stay within the
original [addr, addr + size) range. However, if an unaligned access
affects a single page, we don't need to call tlb_fill() in
store_helper_unaligned() at all, since it would be identical to
the previous tlb_fill() call in store_helper(), and therefore a no-op.
If an unaligned access covers multiple pages, this situation does not
occur.

Therefore simply skip TLB handling in store_helper_unaligned() if we
are dealing with a single page.

Fixes: 2bcf018340cb ("s390x/tcg: low-address protection support")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 accel/tcg/cputlb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
index f90f4312ea..a46f3a654d 100644
--- a/accel/tcg/cputlb.c
+++ b/accel/tcg/cputlb.c
@@ -2248,7 +2248,7 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
     const size_t tlb_off = offsetof(CPUTLBEntry, addr_write);
     uintptr_t index, index2;
     CPUTLBEntry *entry, *entry2;
-    target_ulong page2, tlb_addr, tlb_addr2;
+    target_ulong page1, page2, tlb_addr, tlb_addr2;
     MemOpIdx oi;
     size_t size2;
     int i;
@@ -2256,15 +2256,17 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
     /*
      * Ensure the second page is in the TLB.  Note that the first page
      * is already guaranteed to be filled, and that the second page
-     * cannot evict the first.
+     * cannot evict the first.  An exception to this rule is PAGE_WRITE_INV
+     * handling: the first page could have evicted itself.
      */
+    page1 = addr & TARGET_PAGE_MASK;
     page2 = (addr + size) & TARGET_PAGE_MASK;
     size2 = (addr + size) & ~TARGET_PAGE_MASK;
     index2 = tlb_index(env, mmu_idx, page2);
     entry2 = tlb_entry(env, mmu_idx, page2);
 
     tlb_addr2 = tlb_addr_write(entry2);
-    if (!tlb_hit_page(tlb_addr2, page2)) {
+    if (page1 != page2 && !tlb_hit_page(tlb_addr2, page2)) {
         if (!victim_tlb_hit(env, mmu_idx, index2, tlb_off, page2)) {
             tlb_fill(env_cpu(env), page2, size2, MMU_DATA_STORE,
                      mmu_idx, retaddr);
-- 
2.35.3

[PATCH 2/3] hw/misc: Add mmio-debug-exit device

[Ilya Leoshkevich]

System tests on x86 use isa-debug-exit device in order to signal
success or failure to the test runner. Unfortunately it's not easily
usable on other architectures, since a guest needs to access
address_space_io, which may not be as straightforward as on x86.
Also, it requires adding ISA bus, which an architecture might not
otherwise need.

Introduce mmio-debug-exit device, which has the same semantics, but is
triggered by writes to memory.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 hw/misc/Kconfig          |  3 ++
 hw/misc/debugexit_mmio.c | 80 ++++++++++++++++++++++++++++++++++++++++
 hw/misc/meson.build      |  1 +
 hw/s390x/Kconfig         |  1 +
 4 files changed, 85 insertions(+)
 create mode 100644 hw/misc/debugexit_mmio.c

diff --git a/hw/misc/Kconfig b/hw/misc/Kconfig
index cbabe9f78c..0f12735ef7 100644
--- a/hw/misc/Kconfig
+++ b/hw/misc/Kconfig
@@ -15,6 +15,9 @@ config ISA_DEBUG
     bool
     depends on ISA_BUS
 
+config MMIO_DEBUGEXIT
+    bool
+
 config SGA
     bool
     depends on ISA_BUS
diff --git a/hw/misc/debugexit_mmio.c b/hw/misc/debugexit_mmio.c
new file mode 100644
index 0000000000..5e823cc01c
--- /dev/null
+++ b/hw/misc/debugexit_mmio.c
@@ -0,0 +1,80 @@
+/*
+ * Exit with status X when the guest writes X (little-endian) to a specified
+ * address. For testing purposes only.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "exec/address-spaces.h"
+#include "exec/memory.h"
+#include "hw/qdev-core.h"
+#include "hw/qdev-properties.h"
+
+#define TYPE_MMIO_DEBUG_EXIT_DEVICE "mmio-debug-exit"
+OBJECT_DECLARE_SIMPLE_TYPE(MMIODebugExitState, MMIO_DEBUG_EXIT_DEVICE)
+
+struct MMIODebugExitState {
+    DeviceState parent_obj;
+
+    uint32_t base;
+    uint32_t size;
+    MemoryRegion region;
+};
+
+static uint64_t mmio_debug_exit_read(void *opaque, hwaddr addr, unsigned size)
+{
+    return 0;
+}
+
+static void mmio_debug_exit_write(void *opaque, hwaddr addr, uint64_t val,
+                                  unsigned width)
+{
+    exit(val);
+}
+
+static const MemoryRegionOps mmio_debug_exit_ops = {
+    .read = mmio_debug_exit_read,
+    .write = mmio_debug_exit_write,
+    .valid.min_access_size = 1,
+    .valid.max_access_size = 8,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+};
+
+static void mmio_debug_exit_realizefn(DeviceState *d, Error **errp)
+{
+    MMIODebugExitState *s = MMIO_DEBUG_EXIT_DEVICE(d);
+
+    memory_region_init_io(&s->region, OBJECT(s), &mmio_debug_exit_ops, s,
+                          TYPE_MMIO_DEBUG_EXIT_DEVICE, s->size);
+    memory_region_add_subregion(get_system_memory(), s->base, &s->region);
+}
+
+static Property mmio_debug_exit_properties[] = {
+    DEFINE_PROP_UINT32("base", MMIODebugExitState, base, 0),
+    DEFINE_PROP_UINT32("size", MMIODebugExitState, size, 1),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
+static void mmio_debug_exit_class_initfn(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+
+    dc->realize = mmio_debug_exit_realizefn;
+    device_class_set_props(dc, mmio_debug_exit_properties);
+    set_bit(DEVICE_CATEGORY_MISC, dc->categories);
+}
+
+static const TypeInfo mmio_debug_exit_info = {
+    .name          = TYPE_MMIO_DEBUG_EXIT_DEVICE,
+    .parent        = TYPE_DEVICE,
+    .instance_size = sizeof(MMIODebugExitState),
+    .class_init    = mmio_debug_exit_class_initfn,
+};
+
+static void mmio_debug_exit_register_types(void)
+{
+    type_register_static(&mmio_debug_exit_info);
+}
+
+type_init(mmio_debug_exit_register_types)
diff --git a/hw/misc/meson.build b/hw/misc/meson.build
index 95268eddc0..1d2a1067dc 100644
--- a/hw/misc/meson.build
+++ b/hw/misc/meson.build
@@ -2,6 +2,7 @@ softmmu_ss.add(when: 'CONFIG_APPLESMC', if_true: files('applesmc.c'))
 softmmu_ss.add(when: 'CONFIG_EDU', if_true: files('edu.c'))
 softmmu_ss.add(when: 'CONFIG_FW_CFG_DMA', if_true: files('vmcoreinfo.c'))
 softmmu_ss.add(when: 'CONFIG_ISA_DEBUG', if_true: files('debugexit.c'))
+softmmu_ss.add(when: 'CONFIG_MMIO_DEBUGEXIT', if_true: files('debugexit_mmio.c'))
 softmmu_ss.add(when: 'CONFIG_ISA_TESTDEV', if_true: files('pc-testdev.c'))
 softmmu_ss.add(when: 'CONFIG_PCA9552', if_true: files('pca9552.c'))
 softmmu_ss.add(when: 'CONFIG_PCI_TESTDEV', if_true: files('pci-testdev.c'))
diff --git a/hw/s390x/Kconfig b/hw/s390x/Kconfig
index 5e7d8a2bae..9223715dcc 100644
--- a/hw/s390x/Kconfig
+++ b/hw/s390x/Kconfig
@@ -5,6 +5,7 @@ config S390_CCW_VIRTIO
     imply VFIO_AP
     imply VFIO_CCW
     imply WDT_DIAG288
+    imply MMIO_DEBUGEXIT
     select PCI
     select S390_FLIC
     select SCLPCONSOLE
-- 
2.35.3

[PATCH 3/3] tests/tcg/s390x: Test unaligned accesses to lowcore

[Ilya Leoshkevich]

Add a small test to avoid regressions.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 tests/tcg/s390x/Makefile.softmmu-target |  9 +++++++++
 tests/tcg/s390x/unaligned-lowcore.S     | 24 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 tests/tcg/s390x/Makefile.softmmu-target
 create mode 100644 tests/tcg/s390x/unaligned-lowcore.S

diff --git a/tests/tcg/s390x/Makefile.softmmu-target b/tests/tcg/s390x/Makefile.softmmu-target
new file mode 100644
index 0000000000..fdee4a7616
--- /dev/null
+++ b/tests/tcg/s390x/Makefile.softmmu-target
@@ -0,0 +1,9 @@
+S390X_SRC=$(SRC_PATH)/tests/tcg/s390x
+VPATH+=$(S390X_SRC)
+QEMU_OPTS=-device mmio-debug-exit,base=0x2000 -kernel
+
+%: %.S
+	$(CC) -march=z13 -m64 -nostartfiles -static -Wl,-Ttext=0 \
+		-Wl,--build-id=none $< -o $@
+
+TESTS += unaligned-lowcore
diff --git a/tests/tcg/s390x/unaligned-lowcore.S b/tests/tcg/s390x/unaligned-lowcore.S
new file mode 100644
index 0000000000..2b34c3d5ef
--- /dev/null
+++ b/tests/tcg/s390x/unaligned-lowcore.S
@@ -0,0 +1,24 @@
+    .org 0x1D0                 /* Program new PSW */
+    .quad 0x180000000, _pgm    /* BA, EA */
+    .org 0x200                 /* lowcore padding */
+
+    .globl _start
+_start:
+    lctlg %c0,%c0,_c0
+    vst %v0,_unaligned
+    mviy _exit,0
+
+_pgm:
+    mviy _exit,1
+
+    .align 8
+_c0:
+    .quad 0x10060000           /* lowcore protection, AFP, VX */
+
+    .byte 0
+_unaligned:
+    .octa 0
+
+    .org 0x2000                /* mmio-debug-exit base */
+_exit:
+    .byte 0
-- 
2.35.3

Re: [PATCH 2/3] hw/misc: Add mmio-debug-exit device

[Richard Henderson]

On 7/12/22 00:26, Ilya Leoshkevich wrote:
> System tests on x86 use isa-debug-exit device in order to signal
> success or failure to the test runner. Unfortunately it's not easily
> usable on other architectures, since a guest needs to access
> address_space_io, which may not be as straightforward as on x86.
> Also, it requires adding ISA bus, which an architecture might not
> otherwise need.
> 
> Introduce mmio-debug-exit device, which has the same semantics, but is
> triggered by writes to memory.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

You shouldn't need this for s390x, as there are already (at least) two other paths to 
qemu_system_shutdown_request.

E.g. SIGP, which has a stop option.


r~

Re: [PATCH 1/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[Richard Henderson]

On 7/12/22 00:26, Ilya Leoshkevich wrote:
> If low-address-protection is active, unaligned stores to non-protected
> parts of lowcore lead to protection exceptions. The reason is that in
> such cases tlb_fill() call in store_helper_unaligned() covers
> [0, addr + size) range, which contains the protected portion of
> lowcore. This range is too large.
> 
> The most straightforward fix would be to make sure we stay within the
> original [addr, addr + size) range. However, if an unaligned access
> affects a single page, we don't need to call tlb_fill() in
> store_helper_unaligned() at all, since it would be identical to
> the previous tlb_fill() call in store_helper(), and therefore a no-op.
> If an unaligned access covers multiple pages, this situation does not
> occur.
> 
> Therefore simply skip TLB handling in store_helper_unaligned() if we
> are dealing with a single page.
> 
> Fixes: 2bcf018340cb ("s390x/tcg: low-address protection support")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Queueing to tcg-next.  I'll let the test case go through s390x.


r~

Re: [PATCH 1/3] accel/tcg: Fix unaligned stores to s390x low-address-protected lowcore

[David Hildenbrand]

On 11.07.22 20:56, Ilya Leoshkevich wrote:
> If low-address-protection is active, unaligned stores to non-protected
> parts of lowcore lead to protection exceptions. The reason is that in
> such cases tlb_fill() call in store_helper_unaligned() covers
> [0, addr + size) range, which contains the protected portion of
> lowcore. This range is too large.
> 
> The most straightforward fix would be to make sure we stay within the
> original [addr, addr + size) range. However, if an unaligned access
> affects a single page, we don't need to call tlb_fill() in
> store_helper_unaligned() at all, since it would be identical to
> the previous tlb_fill() call in store_helper(), and therefore a no-op.
> If an unaligned access covers multiple pages, this situation does not
> occur.
> 
> Therefore simply skip TLB handling in store_helper_unaligned() if we
> are dealing with a single page.
> 
> Fixes: 2bcf018340cb ("s390x/tcg: low-address protection support")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  accel/tcg/cputlb.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
> index f90f4312ea..a46f3a654d 100644
> --- a/accel/tcg/cputlb.c
> +++ b/accel/tcg/cputlb.c
> @@ -2248,7 +2248,7 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
>      const size_t tlb_off = offsetof(CPUTLBEntry, addr_write);
>      uintptr_t index, index2;
>      CPUTLBEntry *entry, *entry2;
> -    target_ulong page2, tlb_addr, tlb_addr2;
> +    target_ulong page1, page2, tlb_addr, tlb_addr2;
>      MemOpIdx oi;
>      size_t size2;
>      int i;
> @@ -2256,15 +2256,17 @@ store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
>      /*
>       * Ensure the second page is in the TLB.  Note that the first page
>       * is already guaranteed to be filled, and that the second page
> -     * cannot evict the first.
> +     * cannot evict the first.  An exception to this rule is PAGE_WRITE_INV
> +     * handling: the first page could have evicted itself.
>       */
> +    page1 = addr & TARGET_PAGE_MASK;
>      page2 = (addr + size) & TARGET_PAGE_MASK;
>      size2 = (addr + size) & ~TARGET_PAGE_MASK;
>      index2 = tlb_index(env, mmu_idx, page2);
>      entry2 = tlb_entry(env, mmu_idx, page2);
>  
>      tlb_addr2 = tlb_addr_write(entry2);
> -    if (!tlb_hit_page(tlb_addr2, page2)) {
> +    if (page1 != page2 && !tlb_hit_page(tlb_addr2, page2)) {
>          if (!victim_tlb_hit(env, mmu_idx, index2, tlb_off, page2)) {
>              tlb_fill(env_cpu(env), page2, size2, MMU_DATA_STORE,
>                       mmu_idx, retaddr);

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb

Re: [PATCH 2/3] hw/misc: Add mmio-debug-exit device

[Ilya Leoshkevich]

On Tue, 2022-07-12 at 10:42 +0530, Richard Henderson wrote:
> On 7/12/22 00:26, Ilya Leoshkevich wrote:
> > System tests on x86 use isa-debug-exit device in order to signal
> > success or failure to the test runner. Unfortunately it's not
> > easily
> > usable on other architectures, since a guest needs to access
> > address_space_io, which may not be as straightforward as on x86.
> > Also, it requires adding ISA bus, which an architecture might not
> > otherwise need.
> > 
> > Introduce mmio-debug-exit device, which has the same semantics, but
> > is
> > triggered by writes to memory.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> 
> You shouldn't need this for s390x, as there are already (at least)
> two other paths to 
> qemu_system_shutdown_request.
> 
> E.g. SIGP, which has a stop option.
> 
> 
> r~
> 

I would normally use lpswe + disabled wait, but this always gives me
exit status code 0, which doesn't allow easily distinguishing between
success and failure.

Code-wise SIGP seems to do roughly the same thing, and a quick
experiment with:

    lgfi %r4,-1
    lgfi %r5,-1
    larl %r6,_cpuaddr
    stap 0(%r6)
    lh %r6,0(%r6)
    nilh %r6,0
    sigp %r4,%r6,5
_cpuaddr: .short 0

confirmed that we get exit status code 0 as well.

Best regards,
Ilya

Re: [PATCH 2/3] hw/misc: Add mmio-debug-exit device

[David Hildenbrand]

On 12.07.22 11:52, Ilya Leoshkevich wrote:
> On Tue, 2022-07-12 at 10:42 +0530, Richard Henderson wrote:
>> On 7/12/22 00:26, Ilya Leoshkevich wrote:
>>> System tests on x86 use isa-debug-exit device in order to signal
>>> success or failure to the test runner. Unfortunately it's not
>>> easily
>>> usable on other architectures, since a guest needs to access
>>> address_space_io, which may not be as straightforward as on x86.
>>> Also, it requires adding ISA bus, which an architecture might not
>>> otherwise need.
>>>
>>> Introduce mmio-debug-exit device, which has the same semantics, but
>>> is
>>> triggered by writes to memory.
>>>
>>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>>
>> You shouldn't need this for s390x, as there are already (at least)
>> two other paths to 
>> qemu_system_shutdown_request.
>>
>> E.g. SIGP, which has a stop option.
>>
>>
>> r~
>>
> 
> I would normally use lpswe + disabled wait, but this always gives me
> exit status code 0, which doesn't allow easily distinguishing between
> success and failure.
> 
> Code-wise SIGP seems to do roughly the same thing, and a quick
> experiment with:
> 
>     lgfi %r4,-1
>     lgfi %r5,-1
>     larl %r6,_cpuaddr
>     stap 0(%r6)
>     lh %r6,0(%r6)
>     nilh %r6,0
>     sigp %r4,%r6,5
> _cpuaddr: .short 0
> 
> confirmed that we get exit status code 0 as well.

disabled wait should trigger a qemu_system_guest_panicked().

But "panic_action == PANIC_ACTION_SHUTDOWN" seems to only make
qemu_main_loop() return with main_loop_should_exit() == true.

main/qemu_main will always return 0.

We could return != 0 on guest panic, but not sure if that could break
existing scripts. We'd need a new QEMU toggle for that most probably ...

-- 
Thanks,

David / dhildenb

Re: [PATCH 2/3] hw/misc: Add mmio-debug-exit device

[David Hildenbrand]

On 12.07.22 11:52, Ilya Leoshkevich wrote:
> On Tue, 2022-07-12 at 10:42 +0530, Richard Henderson wrote:
>> On 7/12/22 00:26, Ilya Leoshkevich wrote:
>>> System tests on x86 use isa-debug-exit device in order to signal
>>> success or failure to the test runner. Unfortunately it's not
>>> easily
>>> usable on other architectures, since a guest needs to access
>>> address_space_io, which may not be as straightforward as on x86.
>>> Also, it requires adding ISA bus, which an architecture might not
>>> otherwise need.
>>>
>>> Introduce mmio-debug-exit device, which has the same semantics, but
>>> is
>>> triggered by writes to memory.
>>>
>>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>>
>> You shouldn't need this for s390x, as there are already (at least)
>> two other paths to 
>> qemu_system_shutdown_request.
>>
>> E.g. SIGP, which has a stop option.
>>
>>
>> r~
>>
> 
> I would normally use lpswe + disabled wait, but this always gives me
> exit status code 0, which doesn't allow easily distinguishing between
> success and failure.
> 
> Code-wise SIGP seems to do roughly the same thing, and a quick
> experiment with:
> 
>     lgfi %r4,-1
>     lgfi %r5,-1
>     larl %r6,_cpuaddr
>     stap 0(%r6)
>     lh %r6,0(%r6)
>     nilh %r6,0
>     sigp %r4,%r6,5
> _cpuaddr: .short 0
> 
> confirmed that we get exit status code 0 as well.

disabled wait should trigger a qemu_system_guest_panicked().

But "panic_action == PANIC_ACTION_SHUTDOWN" seems to only make
qemu_main_loop() return with main_loop_should_exit() == true.

main/qemu_main will always return 0.

We could return != 0 on guest panic, but not sure if that could break
existing scripts. We'd need a new QEMU toggle for that most probably ...

-- 
Thanks,

David / dhildenb

Re: [PATCH 2/3] hw/misc: Add mmio-debug-exit device

[Ilya Leoshkevich]

On Tue, 2022-07-12 at 12:08 +0200, David Hildenbrand wrote:
> On 12.07.22 11:52, Ilya Leoshkevich wrote:
> > On Tue, 2022-07-12 at 10:42 +0530, Richard Henderson wrote:
> > > On 7/12/22 00:26, Ilya Leoshkevich wrote:
> > > > System tests on x86 use isa-debug-exit device in order to
> > > > signal
> > > > success or failure to the test runner. Unfortunately it's not
> > > > easily
> > > > usable on other architectures, since a guest needs to access
> > > > address_space_io, which may not be as straightforward as on
> > > > x86.
> > > > Also, it requires adding ISA bus, which an architecture might
> > > > not
> > > > otherwise need.
> > > > 
> > > > Introduce mmio-debug-exit device, which has the same semantics,
> > > > but
> > > > is
> > > > triggered by writes to memory.
> > > > 
> > > > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > > 
> > > You shouldn't need this for s390x, as there are already (at
> > > least)
> > > two other paths to 
> > > qemu_system_shutdown_request.
> > > 
> > > E.g. SIGP, which has a stop option.
> > > 
> > > 
> > > r~
> > > 
> > 
> > I would normally use lpswe + disabled wait, but this always gives
> > me
> > exit status code 0, which doesn't allow easily distinguishing
> > between
> > success and failure.
> > 
> > Code-wise SIGP seems to do roughly the same thing, and a quick
> > experiment with:
> > 
> >     lgfi %r4,-1
> >     lgfi %r5,-1
> >     larl %r6,_cpuaddr
> >     stap 0(%r6)
> >     lh %r6,0(%r6)
> >     nilh %r6,0
> >     sigp %r4,%r6,5
> > _cpuaddr: .short 0
> > 
> > confirmed that we get exit status code 0 as well.
> 
> disabled wait should trigger a qemu_system_guest_panicked().
> 
> But "panic_action == PANIC_ACTION_SHUTDOWN" seems to only make
> qemu_main_loop() return with main_loop_should_exit() == true.
> 
> main/qemu_main will always return 0.
> 
> We could return != 0 on guest panic, but not sure if that could break
> existing scripts. We'd need a new QEMU toggle for that most probably
> ...
> 

I wonder if a device is a cleaner way to solve this? It may be used on
all architectures, so there is no need to invent per-architecture way
to exit with a specific code. Maybe we can even replace Intel's
debugexit with it.