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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jul 03, 2020 at 11:56:38AM +0200, Jan Beulich wrote:
> 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"?

That could work, not sure if it needs to state however that those will
be 4K pages, since Arm can have a different minimum page size IIRC?
(or that's already the assumption for all number of frames fields)
vmtrace_nr_frames seems fine to me.

Roger.