From: Attilio Rao Date: Wed, 22 Aug 2012 15:59:19 +0000 (+0100) Subject: XEN: Document the semantic of x86_init.mapping.pagetable_reserve X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=723545383f2c6867e00c0780dfcc4ecdacf59ab4;p=people%2Fattilio%2Flinux-torvalds.git XEN: Document the semantic of x86_init.mapping.pagetable_reserve The informations added on the hook are: - Native behaviour - Xen specific behaviour - Logic behind the Xen specific behaviour - setup function semantic Even if in general it would be a good idea to separate implementation from semantic, in this case it is beneficial to give a little bit of context on the implementation in order to favourably understand the semantic. Signed-off-by: Attilio Rao Reviewed-by: Konrad Rzeszutek Wilk Acked-by: Stefano Stabellini --- diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h index 38155f66714..b22093cd2a5 100644 --- a/arch/x86/include/asm/x86_init.h +++ b/arch/x86/include/asm/x86_init.h @@ -72,8 +72,23 @@ struct x86_init_oem { * struct x86_init_mapping - platform specific initial kernel pagetable setup * @pagetable_reserve: reserve a range of addresses for kernel pagetable usage * - * For more details on the purpose of this hook, look in - * init_memory_mapping and the commit that added it. + * It does reserve a range of pages, to be used as pagetable pages. + * The start and end parameters are expected to be contained in the + * [pgt_buf_start, pgt_buf_top] range. + * The native implementation reserves the pages via the memblock_reserve() + * interface. + * The Xen implementation, besides reserving the range via memblock_reserve(), + * also sets RW the remaining pages contained in the ranges + * [pgt_buf_start, start) and [end, pgt_buf_top). + * This is needed because the range [pgt_buf_start, pgt_buf_top] was + * previously mapped read-only by xen_set_pte_init: when running + * on Xen all the pagetable pages need to be mapped read-only in order to + * avoid protection faults from the hypervisor. However, once the correct + * amount of pages is reserved for the pagetables, all the others contained + * in the range must be set to RW so that they can be correctly recycled by + * the VM subsystem. + * This operation is meant to be performed only during init_memory_mapping(), + * just after space for the kernel direct mapping tables is found. */ struct x86_init_mapping { void (*pagetable_reserve)(u64 start, u64 end);