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 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.
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!
Kind regards,
Henry

> Jan