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Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to scl
From: |
Thomas Huth |
Subject: |
Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h |
Date: |
Mon, 11 Sep 2017 04:19:03 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
On 08.09.2017 14:29, Markus Armbruster wrote:
> Thomas Huth <address@hidden> writes:
>
>> On 07.09.2017 22:13, David Hildenbrand wrote:
>>> Implemented in sclp.c, so let's move it to the right include file.
>>> Fix up one include. Do a forward declaration of CPUS390XState to fix the
>>> two sclp consoles complaining.
>>>
>>> Signed-off-by: David Hildenbrand <address@hidden>
>>> ---
>>> include/hw/s390x/sclp.h | 2 ++
>>> target/s390x/cpu.h | 1 -
>>> target/s390x/misc_helper.c | 1 +
>>> 3 files changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/hw/s390x/sclp.h b/include/hw/s390x/sclp.h
>>> index a72d096081..4b86a8a293 100644
>>> --- a/include/hw/s390x/sclp.h
>>> +++ b/include/hw/s390x/sclp.h
>>> @@ -242,5 +242,7 @@ sclpMemoryHotplugDev
>>> *init_sclp_memory_hotplug_dev(void);
>>> sclpMemoryHotplugDev *get_sclp_memory_hotplug_dev(void);
>>> void sclp_service_interrupt(uint32_t sccb);
>>> void raise_irq_cpu_hotplug(void);
>>> +typedef struct CPUS390XState CPUS390XState;
>>> +int sclp_service_call(CPUS390XState *env, uint64_t sccb, uint32_t code);
>>
>> That's dangerous and likely does not work with certain versions of GCC.
>> You can't do a "forward declaration" with typedef in C, I'm afraid. See
>> for example:
>>
>> https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg01454.html
>> https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg03337.html
>> https://stackoverflow.com/questions/8367646/redefinition-of-typedef
>>
>> All this typedef'ing in QEMU is pretty bad ... we run into this problem
>> again and again. include/qemu/typedefs.h is just a work-around for this.
>> I know people like typedefs for some reasons (I used to do that, too,
>> before I realized the trouble with them), but IMHO we should rather
>> adopt the typedef-related rules from the kernel coding conventions instead:
>>
>> https://www.kernel.org/doc/html/v4.13/process/coding-style.html#typedefs
>
> I prefer the kernel style for typedefs myself. But it's strictly a
> matter of style. Excessive typedeffing makes code harder to understand,
> it isn't wrong. The part that's wrong is defining things more than
> once, and that applies to everything, not just typedefs.
>
> Sometimes you get away with defining something more than once. It's
> still wrong.
>
> You're not supposed to define the same variable more than once, either
> (although many C compilers let you get away with it, see -fno-common).
> You define it in *one* place. If you need to declare it, declare it in
> *one* place, and make sure that place is in scope at the definition, so
> the compiler can check the two match.
>
> Likewise, you're not supposed to define the same struct tag more than
> once, and you should declare it in just one place.
AFAIK it's perfect valid C to do a forward declaration of a struct
multiple times by just writing e.g. "struct CPUS390XState;" somewhere in
your code. This is also what is done in various Linux kernel headers all
over the place.
> Likewise, you're not supposed to define (with typedef) the same type
> more than once. There is no such thing as a typedef declaration.
>
> qemu/typedefs.h is not a work-around for a typedef-happy style. Its
> purpose is breaking inclusion cycles. Secondary purpose is reducing the
> need for non-cyclic includes. If we didn't typedef, we'd still put our
> struct declarations there.
No, since it's not required for struct forward declarations, only to
avoid multiple typedef definitions.
Thomas
- [Qemu-devel] [PATCH v3 04/21] s390x: rename s390-virtio.h to s390-virtio-hcall.h, (continued)
- [Qemu-devel] [PATCH v3 04/21] s390x: rename s390-virtio.h to s390-virtio-hcall.h, David Hildenbrand, 2017/09/07
- [Qemu-devel] [PATCH v3 03/21] s390x: get rid of s390-virtio.c, David Hildenbrand, 2017/09/07
- [Qemu-devel] [PATCH v3 05/21] target/s390x: move typedef of S390CPU to its definition, David Hildenbrand, 2017/09/07
- [Qemu-devel] [PATCH v3 06/21] s390x: move s390_virtio_hypercall() to s390-virtio-hcall.h, David Hildenbrand, 2017/09/07
- [Qemu-devel] [PATCH v3 07/21] s390x: move subsystem_reset() to s390-virtio-ccw.h, David Hildenbrand, 2017/09/07
- [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, David Hildenbrand, 2017/09/07
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Thomas Huth, 2017/09/08
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, David Hildenbrand, 2017/09/08
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Eduardo Habkost, 2017/09/09
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Thomas Huth, 2017/09/10
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Eduardo Habkost, 2017/09/11
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Paolo Bonzini, 2017/09/11
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, David Hildenbrand, 2017/09/11
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Eduardo Habkost, 2017/09/11
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, David Hildenbrand, 2017/09/11
- Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h, Eduardo Habkost, 2017/09/11
[Qemu-devel] [PATCH v3 09/21] target/s390x: use trigger_pgm_exception() in s390_cpu_handle_mmu_fault(), David Hildenbrand, 2017/09/07