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

[Jan Beulich <jbeulich@xxxxxxxx>]

I'm afraid the title doesn't really say what the patch actually means
to achieve.

On 08.03.2024 02:54, Henry Wang wrote:
> Previous commits enable the toolstack to get the domain memory map,
> therefore instead of hardcoding the guest magic pages region, use
> the XEN_DOMCTL_get_mem_map domctl to get the start address of the
> guest magic pages region. Add the (XEN)MEMF_force_heap_alloc memory
> flags to force populate_physmap() to allocate page from domheap
> instead of using 1:1 or static allocated pages to map the magic pages.

A patch description wants to be (largely) self-contained. "Previous
commits" shouldn't be mentioned; recall that the sequence in which
patches go in is unknown to you up front. (In fact the terms "commit"
or "patch" should be avoided altogether when describing what a patch
does. The only valid use I can think of is when referring to commits
already in the tree, and then typically by quoting their hash and
title.)

> --- a/xen/include/public/memory.h
> +++ b/xen/include/public/memory.h
> @@ -41,6 +41,11 @@
>  #define XENMEMF_exact_node(n) (XENMEMF_node(n) | XENMEMF_exact_node_request)
>  /* Flag to indicate the node specified is virtual node */
>  #define XENMEMF_vnode  (1<<18)
> +/*
> + * 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?

Jan