Re: [PATCH gnumach] Enable MACH_HOST and fix non-addressable bitfields

[Damien Zammit]

Hi,

On 2/11/24 10:07 PM, Samuel Thibault wrote:

> Damien Zammit, le dim. 11 févr. 2024 10:55:26 +0000, a ecrit:
>>>> diff --git a/kern/task.h b/kern/task.h
>>>> index dec3a530..27970620 100644
>>>> --- a/kern/task.h
>>>> +++ b/kern/task.h
>>>> @@ -61,11 +61,11 @@ struct task {
>>>>            decl_simple_lock_data(,lock)    /* Task's lock */
>>>>            int             ref_count;      /* Number of references to me */
>>>>
>>>> -  /* Flags */
>>>> -  unsigned int    active:1,       /* Task has not been terminated */
>>>> -  /* boolean_t */ may_assign:1,   /* can assigned pset be changed? */
>>>> -                  assign_active:1,        /* waiting for may_assign */
>>>> -                  essential:1;    /* Is this task essential for the 
>>>> system? */
>>>> +  /* Addressable flags */
>>>> +  unsigned char   active;         /* Task has not been terminated */
>>>> +  unsigned char   may_assign;     /* can assigned pset be changed? */
>>>> +  unsigned char   assign_active;  /* waiting for may_assign */
>>>> +  unsigned char   essential;      /* Is this task essential for the 
>>>> system? */
>>> AIUI only assign_active need to be adressable? Better make only that one
>>> addressable.
>> Looking at the existing flag types, it needs to fit into part of a cacheline.
> Why would it need to fit in a cacheline?

See comment above struct task:

  /*
   * Task name buffer size.  The size is chosen so that struct task fits
   * into three cache lines.  The size of a cache line on a typical CPU
   * is 64 bytes.
   */
  #define TASK_NAME_SIZE 32

>> The way I have done it, I rearranged the existing 4 byte integer to be 4 
>> separate single byte flags.
>> If you want me to put only one of the values into an addressable field,
>> how will I retain the same size for the rest of the fields?
> What do you mean by "the same size"?

I think the actual sizeof(struct task) and its layout matters.

Therefore, my original code change here preserves most of the layout and 
sizeof(struct task).

Damien