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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

Jan