Re: [Qemu-devel] [PATCH] memory: Set notdirty_mem_ops validator

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 9/6/19 3:08 PM, Eric Blake wrote:
> On 9/6/19 3:28 AM, Philippe Mathieu-Daudé wrote:
>> On 9/2/19 3:26 AM, Tony Nguyen wrote:
>>> Existing read rejecting validator was mistakenly cleared.
>>>
>>> Reads dispatched to io_mem_notdirty then segfaults as there is no read
>>> handler.
>>>
>>> Signed-off-by: Tony Nguyen <tony.nguyen@bt.com>
>>> ---
>>>  exec.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/exec.c b/exec.c
>>> index 1df966d17a..05d664541f 100644
>>> --- a/exec.c
>>> +++ b/exec.c
>>> @@ -2796,12 +2796,12 @@ static bool notdirty_mem_accepts(void *opaque, hwaddr addr,
>>>  
>>>  static const MemoryRegionOps notdirty_mem_ops = {
>>>      .write = notdirty_mem_write,
>>> -    .valid.accepts = notdirty_mem_accepts,
>>>      .endianness = DEVICE_NATIVE_ENDIAN,
>>>      .valid = {
>>>          .min_access_size = 1,
>>>          .max_access_size = 8,
>>>          .unaligned = false,
>>> +        .accepts = notdirty_mem_accepts,
>>
>> I'm surprised the compiler doesn't emit any warning...
> 
> Same here.
> 
> But reading
> https://en.cppreference.com/w/c/language/struct_initialization, this is
> compliant behavior:
> 
> "However, when an initializer begins with a left open brace, its current
> object is fully re-initialized and any prior explicit initializers for
> any of its subobjects are ignored:"
> 
> so it is worth filing a gcc bug asking for a QoI improvement in adding a
> warning (since the code does not violate the C standard, but does cause
> surprises in the reinitialization of omitted members in the later {} to
> go back to 0 in spite of the earlier initialization by nested name).

Just remembered another case of (correct) reinitialization in
hw/arm/palm.c:101:

static struct {
    int row;
    int column;
} palmte_keymap[0x80] = {
    [0 ... 0x7f] = { -1, -1 },
    [0x3b] = { 0, 0 },	/* F1	-> Calendar */
    [0x3c] = { 1, 0 },	/* F2	-> Contacts */
    [0x3d] = { 2, 0 },	/* F3	-> Tasks List */
    [0x3e] = { 3, 0 },	/* F4	-> Note Pad */
    [0x01] = { 4, 0 },	/* Esc	-> Power */
    [0x4b] = { 0, 1 },	/* 	   Left */
    [0x50] = { 1, 1 },	/* 	   Down */
    [0x48] = { 2, 1 },	/*	   Up */
    [0x4d] = { 3, 1 },	/*	   Right */
    [0x4c] = { 4, 1 },	/* 	   Centre */
    [0x39] = { 4, 1 },	/* Spc	-> Centre */
};