Re: [Xen-devel] [PATCH V2 2/3] xen/vm_event: Support for guest-requested events

["Lengyel, Tamas" <tlengyel@xxxxxxxxxxx>]

On Mon, Jul 6, 2015 at 6:26 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:

>>> On 30.06.15 at 16:48, <tlengyel@xxxxxxxxxxx> wrote:
>> > --- a/xen/include/asm-x86/domain.h
>
>> >> +++ b/xen/include/asm-x86/domain.h
>> >> @@ -342,13 +342,15 @@ struct arch_domain
>> >>
>> >>      /* Monitor options */
>> >>      struct {
>> >> -        uint16_t write_ctrlreg_enabled       : 4;
>> >> -        uint16_t write_ctrlreg_sync          : 4;
>> >> -        uint16_t write_ctrlreg_onchangeonly  : 4;
>> >> -        uint16_t mov_to_msr_enabled          : 1;
>> >> -        uint16_t mov_to_msr_extended         : 1;
>> >> -        uint16_t singlestep_enabled          : 1;
>> >> -        uint16_t software_breakpoint_enabled : 1;
>> >> +        uint32_t write_ctrlreg_enabled       : 4;
>> >> +        uint32_t write_ctrlreg_sync          : 4;
>> >> +        uint32_t write_ctrlreg_onchangeonly  : 4;
>> >> +        uint32_t mov_to_msr_enabled          : 1;
>> >> +        uint32_t mov_to_msr_extended         : 1;
>> >> +        uint32_t singlestep_enabled          : 1;
>> >> +        uint32_t software_breakpoint_enabled : 1;
>> >> +        uint32_t request_enabled             : 1;
>> >> +        uint32_t request_sync                : 1;
>> >
>> > Can you please switch to plain unsigned int if you already have to
>> > touch this? There's no reason I can see to use a fixed width integer
>> > type here.
>>
>> Ack, will make it plain int.
>
>
> IMHO having it fix-width is easier to read when adding new elements to see
> how many bits we have left free. I would not want this changed unless there
> is a clear benefit to doing so.

I can't see what benefit using fixed widht types here is: Best
demonstrated with uint64_t, gcc doesn't honor the type of the
field anyway. And hence counting the number of left bits (with
some unknown definition of "left") is quite pointless too - it's an
internal structure, and when a new bit field needs to be added,
it doesn't really matter whether it grows the structure (at least
no more than any other field addition).

Bottom line - no fixed width types when not really warranted,
please.

Jan

SGTM.

Thanks,

Tamas

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