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

From: Henry Wang <xin.wang2@amd.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Wei Liu <wl@xen.org>, Anthony PERARD <anthony.perard@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	Alec Kwapis <alec.kwapis@medtronic.com>,
	<xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH v2 5/5] xen/memory, tools: Make init-dom0less consume XEN_DOMCTL_get_mem_map
Date: Tue, 2 Apr 2024 17:03:45 +0800	[thread overview]
Message-ID: <269e2df4-dee9-4863-8888-f23516e2e833@amd.com> (raw)
In-Reply-To: <133d81d2-9ab1-446b-918f-d86e86833cd5@suse.com>

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