Re: [PATCH v2 5/5] xen/memory, tools: Make init-dom0less consume XEN_DOMCTL_get_mem_map

[Henry Wang <xin.wang2@xxxxxxx>]

Hi Jan,

On 4/2/2024 4:51 PM, Jan Beulich wrote:
> On 02.04.2024 10:43, Henry Wang wrote:
> > On 4/2/2024 3:05 PM, Jan Beulich wrote:
> > > On 29.03.2024 06:11, Henry Wang wrote:
> > > > On 3/12/2024 1:07 AM, Jan Beulich wrote:
> > > > > > +/*
> > > > > > + * Flag to force populate physmap to use pages from domheap instead of 1:1
> > > > > > + * or static allocation.
> > > > > > + */
> > > > > > +#define XENMEMF_force_heap_alloc  (1<<19)
> > > > > >     #endif
> > > > > If this is for populate_physmap only, then other sub-ops need to reject
> > > > > its use.
> > > > >
> > > > > I have to admit I'm a little wary of allocating another flag here and ...
> > > > >
> > > > > > --- a/xen/include/xen/mm.h
> > > > > > +++ b/xen/include/xen/mm.h
> > > > > > @@ -205,6 +205,8 @@ struct npfec {
> > > > > >     #define  MEMF_no_icache_flush (1U<<_MEMF_no_icache_flush)
> > > > > >     #define _MEMF_no_scrub    8
> > > > > >     #define  MEMF_no_scrub    (1U<<_MEMF_no_scrub)
> > > > > > +#define _MEMF_force_heap_alloc 9
> > > > > > +#define  MEMF_force_heap_alloc (1U<<_MEMF_force_heap_alloc)
> > > > > >     #define _MEMF_node        16
> > > > > >     #define  MEMF_node_mask   ((1U << (8 * sizeof(nodeid_t))) - 1)
> > > > > >     #define  MEMF_node(n)     ((((n) + 1) & MEMF_node_mask) << _MEMF_node)
> > > > > ... here - we don't have that many left. Since other sub-ops aren't
> > > > > intended to support this flag, did you consider adding another (perhaps
> > > > > even arch-specific) sub-op instead?
> > > > While revisiting this comment when trying to come up with a V3, I
> > > > realized adding a sub-op here in the same level as
> > > > XENMEM_populate_physmap will basically duplicate the function
> > > > populate_physmap() with just the "else" (the non-1:1 allocation) part,
> > > > also a similar xc_domain_populate_physmap_exact() & co will be needed
> > > > from the toolstack side to call the new sub-op. So I am having the
> > > > concern of the duplication of code and not sure if I understand you
> > > > correctly. Would you please elaborate a bit more or clarify if I
> > > > understand you correctly? Thanks!
> > > Well, the goal is to avoid both code duplication and introduction of a new,
> > > single-use flag. The new sub-op suggestion, I realize now, would mainly have
> > > helped with avoiding the new flag in the public interface. That's still
> > > desirable imo. Internally, have you checked which MEMF_* are actually used
> > > by populate_physmap()? Briefly looking, e.g. MEMF_no_dma and MEMF_no_refcount
> > > aren't. It therefore would be possible to consider re-purposing one that
> > > isn't (likely to be) used there. Of course doing so requires care to avoid
> > > passing that flag down to other code (page_alloc.c functions in particular),
> > > where the meaning would be the original one.
> > I think you made a good point, however, to be honest I am not sure about
> > the repurposing flags such as MEMF_no_dma and MEMF_no_refcount, because
> > I think the name and the purpose of the flag should be clear and
> > less-misleading. Reusing either one for another meaning, namely forcing
> > a non-heap allocation in populate_physmap() would be confusing in the
> > future. Also if one day these flags will be needed in
> > populate_physmap(), current repurposing approach will lead to a even
> > confusing code base.
> For the latter - hence "(likely to be)" in my earlier reply.
Agreed.

> For the naming - of course an aliasing #define ought to be used then, to
> make the purpose clear at the use sites.
Well I have to admit the alias #define approach is clever (thanks) and I 
am getting persuaded gradually, as there will be also another benefit 
for me to do less modification from my side :) I will firstly try this 
approach in v3 if ...
> > I also do agree very much that we need to also avoid the code
> > duplication, so compared to other two suggested approach, adding a new
> > MEMF would be the cleanest solution IMHO, as it is just one bit and MEMF
> > flags are not added very often.
> >
> > I would also curious what the other common code maintainers will say on
> > this issue: @Andrew, @Stefano, @Julien, any ideas? Thanks!
...not receiving any other input, and we can continue the discussion in 
v3. Thanks!
Kind regards,
Henry

> Jan