Re: [Xen-devel] [PATCHv6 5/5] gnttab: use per-VCPU maptrack free lists

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.04.15 at 11:09, <malcolm.crossley@xxxxxxxxxx> wrote:
> On 23/04/15 17:11, Jan Beulich wrote:
>>>>> On 22.04.15 at 18:00, <david.vrabel@xxxxxxxxxx> wrote:
>>> --- a/xen/include/xen/grant_table.h
>>> +++ b/xen/include/xen/grant_table.h
>>> @@ -60,6 +60,8 @@ struct grant_mapping {
>>>      u32      ref;           /* grant ref */
>>>      u16      flags;         /* 0-4: GNTMAP_* ; 5-15: unused */
>>>      domid_t  domid;         /* granting domain */
>>> +    u32      vcpu;          /* vcpu which created the grant mapping */
>>> +    u16      pad[2];
>>>  };
>> 
>> What is this pad[] good for?
> 
> The pad is to keep the struct power of 2 sized because this allows the
> compiler to optimise these macro's to right and left shifts:
> 
> #define MAPTRACK_PER_PAGE (PAGE_SIZE / sizeof(struct grant_mapping))
> #define maptrack_entry(t, e) \
>     ((t)->maptrack[(e)/MAPTRACK_PER_PAGE][(e)%MAPTRACK_PER_PAGE])

Okay, then why u16[2] instead of u32? And please add a brief
comment explaining the reason.

Apart from that I wonder whether fitting vcpu in the 10 unused
flags bits (not 11, as the comment on the field suggests) would be
an option. That would require limiting vCPU count to 4k, which I
don't think would really be a problem for anyone.

Jan


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