Re: [Qemu-devel] [PATCH] cpu_defs: Simplify CPUTLB padding logic

[Peter Crosthwaite]

On Mon, Jul 6, 2015 at 4:52 AM, Paolo Bonzini <address@hidden> wrote:
>
>
> On 06/07/2015 13:42, Richard Henderson wrote:
>> On 07/06/2015 09:43 AM, Paolo Bonzini wrote:
>>>
>>>
>>> On 05/07/2015 23:08, Peter Crosthwaite wrote:
>>>> There was a complicated subtractive arithmetic for determining the
>>>> padding on the CPUTLBEntry structure. Simplify this with a union.
>>>>
>>>> Signed-off-by: Peter Crosthwaite <address@hidden>
>>>> ---
>>>>   include/exec/cpu-defs.h | 23 ++++++++++++-----------
>>>>   1 file changed, 12 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
>>>> index 98b9cff..5093be2 100644
>>>> --- a/include/exec/cpu-defs.h
>>>> +++ b/include/exec/cpu-defs.h
>>>> @@ -105,17 +105,18 @@ typedef struct CPUTLBEntry {
>>>>          bit 3                      : indicates that the entry is
>>>> invalid
>>>>          bit 2..0                   : zero
>>>>       */
>>>> -    target_ulong addr_read;
>>>> -    target_ulong addr_write;
>>>> -    target_ulong addr_code;
>>>> -    /* Addend to virtual address to get host address.  IO accesses
>>>> -       use the corresponding iotlb value.  */
>>>> -    uintptr_t addend;
>>>> -    /* padding to get a power of two size */
>>>> -    uint8_t dummy[(1 << CPU_TLB_ENTRY_BITS) -
>>>> -                  (sizeof(target_ulong) * 3 +
>>>> -                   ((-sizeof(target_ulong) * 3) & (sizeof(uintptr_t)
>>>> - 1)) +
>>>> -                   sizeof(uintptr_t))];
>>>> +    union {
>>>
>>> The struct CPUTLBEntry can be changed to union CPUTLBEntry directly,
>>> with no need for the anonymous struct.
>>
>> Um, no it can't.  That would put all of the members at the same address.
>
> Of course. :-(  With no need for the anonymous _union_.  *blush*.
>

Yeh this is what I assumed you meant. You still need one anonymous
struct, but it saves one level of indent and one less anonymous thing.

Regards,
Peter

>>> Which compiler version started implementing anonymous structs?
>>
>> A long long time ago -- gcc 2 era.
>
> Great.  I now remember that the recent feature is anonymous tagged
> structs, coming from the Plan 9 compiler.
>
> Paolo
>
>>> Or can we just add
>>>
>>>      __attribute__((__aligned__(1 << CPU_TLB_ENTRY_BITS)))
>>
>> The structure isn't currently aligned, and it needn't be.  We only need
>> the size to be a power of two for the addressing.
>>
>>
>>
>> r~
>