Re: [PATCH v4 03/10] tools/libxl: add vmtrace_pt_size parameter

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.07.2020 11:44, Roger Pau Monné wrote:
> On Thu, Jul 02, 2020 at 06:23:28PM +0200, Michał Leszczyński wrote:
>> ----- 2 lip 2020 o 11:00, Roger Pau Monné roger.pau@xxxxxxxxxx napisał(a):
>>
>>> On Tue, Jun 30, 2020 at 02:33:46PM +0200, Michał Leszczyński wrote:
>>>> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
>>>> index 59bdc28c89..7b8289d436 100644
>>>> --- a/xen/include/public/domctl.h
>>>> +++ b/xen/include/public/domctl.h
>>>> @@ -92,6 +92,7 @@ struct xen_domctl_createdomain {
>>>>      uint32_t max_evtchn_port;
>>>>      int32_t max_grant_frames;
>>>>      int32_t max_maptrack_frames;
>>>> +    uint8_t vmtrace_pt_order;
>>>
>>> I've been thinking about this, and even though this is a domctl (so
>>> not a stable interface) we might want to consider using a size (or a
>>> number of pages) here rather than an order. IPT also supports
>>> TOPA mode (kind of a linked list of buffers) that would allow for
>>> sizes not rounded to order boundaries to be used, since then only each
>>> item in the linked list needs to be rounded to an order boundary, so
>>> you could for example use three 4K pages in TOPA mode AFAICT.
>>>
>>> Roger.
>>
>> In previous versions it was "size" but it was requested to change it
>> to "order" in order to shrink the variable size from uint64_t to
>> uint8_t, because there is limited space for xen_domctl_createdomain
>> structure.
> 
> It's likely I'm missing something here, but I wasn't aware
> xen_domctl_createdomain had any constrains regarding it's size. It's
> currently 48bytes which seems fairly small.

Additionally I would guess a uint32_t could do here, if the value
passed was "number of pages" rather than "number of bytes"?

Jan