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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

For the naming - of course an aliasing #define ought to be used then, to
make the purpose clear at the use sites.

Jan

> 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