Re: [PATCH V3 (resend) 11/19] x86/setup: Leave early boot slightly earlier

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, May 13, 2024 at 01:40:38PM +0000, Elias El Yandouzi wrote:
> From: Hongyan Xia <hongyxia@xxxxxxxxxx>
> 
> When we do not have a direct map, memory for metadata of heap nodes in
> init_node_heap() is allocated from xenheap, which needs to be mapped and
> unmapped on demand. However, we cannot just take memory from the boot
> allocator to create the PTEs while we are passing memory to the heap
> allocator.
> 
> To solve this race, we leave early boot slightly sooner so that Xen PTE
> pages are allocated from the heap instead of the boot allocator. We can
> do this because the metadata for the 1st node is statically allocated,
> and by the time we need memory to create mappings for the 2nd node, we
> already have enough memory in the heap allocator in the 1st node.
> 
> Signed-off-by: Hongyan Xia <hongyxia@xxxxxxxxxx>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> Signed-off-by: Elias El Yandouzi <eliasely@xxxxxxxxxx>
> 
> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> index bd6b1184f5..f26c9799e4 100644
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -1751,6 +1751,22 @@ void asmlinkage __init noreturn __start_xen(unsigned 
> long mbi_p)
>  
>      numa_initmem_init(0, raw_max_page);
>  
> +    /*
> +     * When we do not have a direct map, memory for metadata of heap nodes in
> +     * init_node_heap() is allocated from xenheap, which needs to be mapped 
> and
> +     * unmapped on demand.

Hm, maybe I'm confused, but isn't xenheap memory supposed to be always
mapped when in use?  In one of the previous patches xenheap memory is
unconditionally mapped in alloc_xenheap_pages().

IMO, this would better be worded as:  "... is allocated from xenheap,
which needs to be mapped at allocation and unmapped when freed."

>                             However, we cannot just take memory from the boot
> +     * allocator to create the PTEs while we are passing memory to the heap
> +     * allocator during end_boot_allocator().

Could you elaborate here, I don't obviously see why we can't consume
memory from the boot allocator.  Is it because under certain
conditions we might try to allocate memory from the boot allocator in
order to fulfill a call to map_pages_to_xen() and find the boot
allocator empty?

Thanks, Roger.