Re: [Xen-devel] [PATCH v1] getpageframeinfo3: replace hardcoded batchsize with constant

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/03/18 16:49, George Dunlap wrote:
> On 03/09/2018 04:43 PM, Jan Beulich wrote:
>>>>> On 09.03.18 at 17:30, <george.dunlap@xxxxxxxxxx> wrote:
>>> On 03/09/2018 04:25 PM, Jan Beulich wrote:
>>>>>>> On 09.03.18 at 17:17, <olaf@xxxxxxxxx> wrote:
>>>>> --- a/xen/include/public/domctl.h
>>>>> +++ b/xen/include/public/domctl.h
>>>>> @@ -137,6 +137,8 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_getdomaininfo_t);
>>>>>  #define XEN_DOMCTL_PFINFO_BROKEN  (0xdU<<28) /* broken page */
>>>>>  #define XEN_DOMCTL_PFINFO_LTAB_MASK (0xfU<<28)
>>>>>  
>>>>> +#define XEN_GETPAGEFRAMEINFO3_MAX_SIZE 1024U
>>>> This is an implementation detail; it shouldn't be made part of the
>>>> public interface. If there's a need for user land to know the value,
>>>> and if there's currently no way to query it, that's what you
>>>> would want to add.
>>> But the domctl interface isn't stable, right?  There's no need to make a
>>> flexible backwards-compatible interface between libxc and Xen; sharing a
>>> #define should be fine, as long as there's only one place to change it.
>> Well, strictly speaking this is an option. But I would prefer if we didn't
>> abuse the "is not a stable interface" property, which this changes
>> feels like it would. 
>>
>> Furthermore it hasn't become clear to me why, if this hard coded
>> number is deemed a problem, we don't get rid of it altogether.
>> Domctl-s have long gained the ability to be preemptible - there
>> various examples.
> Yes, making it preemptible and removing the limit is probably a better
> option.

The 1024 limit was from the 32bit days, where there was a single domheap
page allocated as a bounce buffer.

The logic nowadays is trivial to make continuable, and would remove the
fixed arbitrary upper limit.

~Andrew

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