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[Xen-devel] [PATCH] linux/i386: make kernel tolerate variable start of hypervisor hole

This is the kernel side of the needed change. The hypervisor side, as it's
useful only with 32-on-64, will come with those patches. On an 8 Gb
machine, I saw a lowmem increase of 160 Mb.
I didn't go the capabilities route, but rather introduced a separate note
that specifies the lower boundary the kernel wants to tolerate for the
hypervisor hole. That way one might one day support more than 16 Gb
for x86-32/PAE (at the obvious price of further reducing lowmem in the
guest).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

Index: head-2006-11-06/arch/i386/kernel/head-xen.S
===================================================================
--- head-2006-11-06.orig/arch/i386/kernel/head-xen.S    2006-11-06 
09:07:40.000000000 +0100
+++ head-2006-11-06/arch/i386/kernel/head-xen.S 2006-11-10 16:38:48.000000000 
+0100
@@ -9,7 +9,7 @@
 #include <asm/page.h>
 #include <asm/thread_info.h>
 #include <asm/asm-offsets.h>
-#include <xen/interface/arch-x86_32.h>
+#include <xen/interface/xen.h>
 #include <xen/interface/elfnote.h>
 
 /*
@@ -192,6 +192,7 @@ ENTRY(cpu_gdt_table)
 #endif /* !CONFIG_XEN_COMPAT_030002 */
        ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          .long,  startup_32)
        ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .long,  hypercall_page)
+       ELFNOTE(Xen, XEN_ELFNOTE_HV_START_LOW,   .long,  HYPERVISOR_VIRT_START)
        ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .asciz,
"writable_page_tables|writable_descriptor_tables|auto_translated_physmap|pae_pgdir_above_4gb|supervisor_mode_kernel")
 #ifdef CONFIG_X86_PAE
        ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE,       .asciz, "yes")
Index: head-2006-11-06/include/asm-i386/mach-xen/setup_arch_post.h
===================================================================
--- head-2006-11-06.orig/include/asm-i386/mach-xen/setup_arch_post.h    
2006-10-30 18:05:24.000000000 +0100
+++ head-2006-11-06/include/asm-i386/mach-xen/setup_arch_post.h 2006-11-10 
15:43:02.000000000 +0100
@@ -92,8 +92,10 @@ void __init machine_specific_arch_setup(
 #endif
 
        if (HYPERVISOR_xen_version(XENVER_platform_parameters,
-                                  &pp) == 0)
-               set_fixaddr_top(pp.virt_start - PAGE_SIZE);
+                                  &pp) == 0) {
+               hypervisor_virt_start = pp.virt_start;
+               set_fixaddr_top();
+       }
 
        machine_to_phys_mapping = (unsigned long *)MACH2PHYS_VIRT_START;
        machine_to_phys_nr_ents = MACH2PHYS_NR_ENTRIES;
Index: head-2006-11-06/arch/i386/mm/fault-xen.c
===================================================================
--- head-2006-11-06.orig/arch/i386/mm/fault-xen.c       2006-10-30 
13:40:28.000000000 +0100
+++ head-2006-11-06/arch/i386/mm/fault-xen.c    2006-11-10 15:38:42.000000000 
+0100
@@ -282,12 +282,6 @@ static int spurious_fault(struct pt_regs
        pmd_t *pmd;
        pte_t *pte;
 
-#ifdef CONFIG_XEN
-       /* Faults in hypervisor area are never spurious. */
-       if (address >= HYPERVISOR_VIRT_START)
-               return 0;
-#endif
-
        /* Reserved-bit violation or user access to kernel space? */
        if (error_code & 0x0c)
                return 0;
@@ -372,7 +366,7 @@ fastcall void __kprobes do_page_fault(st
        if (unlikely(address >= TASK_SIZE)) { 
 #ifdef CONFIG_XEN
                /* Faults in hypervisor area can never be patched up. */
-               if (address >= HYPERVISOR_VIRT_START)
+               if (address >= hypervisor_virt_start)
                        goto bad_area_nosemaphore;
 #endif
                if (!(error_code & 5))
Index: head-2006-11-06/arch/i386/mm/init-xen.c
===================================================================
--- head-2006-11-06.orig/arch/i386/mm/init-xen.c        2006-10-04 
10:25:54.000000000 +0200
+++ head-2006-11-06/arch/i386/mm/init-xen.c     2006-11-10 15:49:49.000000000 
+0100
@@ -130,7 +130,7 @@ static void __init page_table_range_init
                pud = pud_offset(pgd, vaddr);
                pmd = pmd_offset(pud, vaddr);
                for (; (pmd_idx < PTRS_PER_PMD) && (vaddr != end); pmd++, 
pmd_idx++) {
-                       if (vaddr < HYPERVISOR_VIRT_START && pmd_none(*pmd)) 
+                       if (vaddr < hypervisor_virt_start && pmd_none(*pmd))
                                one_page_table_init(pmd);
 
                        vaddr += PMD_SIZE;
@@ -187,7 +187,7 @@ static void __init kernel_physical_mappi
                pmd += pmd_idx;
                for (; pmd_idx < PTRS_PER_PMD && pfn < max_low_pfn; pmd++, 
pmd_idx++) {
                        unsigned int address = pfn * PAGE_SIZE + PAGE_OFFSET;
-                       if (address >= HYPERVISOR_VIRT_START)
+                       if (address >= hypervisor_virt_start)
                                continue;
 
                        /* Map with big pages if possible, otherwise create 
normal page tables. */
@@ -410,7 +410,7 @@ static void __init pagetable_init (void)
         * created - mappings will be set by set_fixmap():
         */
        vaddr = __fix_to_virt(__end_of_fixed_addresses - 1) & PMD_MASK;
-       page_table_range_init(vaddr, 0, pgd_base);
+       page_table_range_init(vaddr, hypervisor_virt_start, pgd_base);
 
        permanent_kmaps_init(pgd_base);
 }
Index: head-2006-11-06/arch/i386/mm/pgtable-xen.c
===================================================================
--- head-2006-11-06.orig/arch/i386/mm/pgtable-xen.c     2006-11-06 
12:22:54.000000000 +0100
+++ head-2006-11-06/arch/i386/mm/pgtable-xen.c  2006-11-10 15:49:08.000000000 
+0100
@@ -186,9 +186,16 @@ void set_pmd_pfn(unsigned long vaddr, un
 }
 
 static int nr_fixmaps = 0;
+unsigned long hypervisor_virt_start = HYPERVISOR_VIRT_START;
 unsigned long __FIXADDR_TOP = (HYPERVISOR_VIRT_START - 2 * PAGE_SIZE);
 EXPORT_SYMBOL(__FIXADDR_TOP);
 
+void __init set_fixaddr_top()
+{
+       BUG_ON(nr_fixmaps > 0);
+       __FIXADDR_TOP = hypervisor_virt_start - 2 * PAGE_SIZE;
+}
+
 void __set_fixmap (enum fixed_addresses idx, maddr_t phys, pgprot_t flags)
 {
        unsigned long address = __fix_to_virt(idx);
@@ -211,12 +218,6 @@ void __set_fixmap (enum fixed_addresses 
        nr_fixmaps++;
 }
 
-void set_fixaddr_top(unsigned long top)
-{
-       BUG_ON(nr_fixmaps > 0);
-       __FIXADDR_TOP = top - PAGE_SIZE;
-}
-
 pte_t *pte_alloc_one_kernel(struct mm_struct *mm, unsigned long address)
 {
        pte_t *pte = (pte_t 
*)__get_free_page(GFP_KERNEL|__GFP_REPEAT|__GFP_ZERO);
Index: head-2006-11-06/include/asm-i386/mach-xen/asm/fixmap.h
===================================================================
--- head-2006-11-06.orig/include/asm-i386/mach-xen/asm/fixmap.h 2006-11-03 
10:30:39.000000000 +0100
+++ head-2006-11-06/include/asm-i386/mach-xen/asm/fixmap.h      2006-11-10 
15:46:13.000000000 +0100
@@ -98,7 +98,7 @@ enum fixed_addresses {
 extern void __set_fixmap(enum fixed_addresses idx,
                                        maddr_t phys, pgprot_t flags);
 
-extern void set_fixaddr_top(unsigned long top);
+extern void set_fixaddr_top(void);
 
 #define set_fixmap(idx, phys) \
                __set_fixmap(idx, phys, PAGE_KERNEL)
Index: head-2006-11-06/include/asm-i386/mach-xen/asm/hypervisor.h
===================================================================
--- head-2006-11-06.orig/include/asm-i386/mach-xen/asm/hypervisor.h     
2006-10-26 11:42:38.000000000 +0200
+++ head-2006-11-06/include/asm-i386/mach-xen/asm/hypervisor.h  2006-11-10 
15:48:03.000000000 +0100
@@ -56,6 +56,10 @@
 
 extern shared_info_t *HYPERVISOR_shared_info;
 
+#ifdef CONFIG_X86_32
+extern unsigned long hypervisor_virt_start;
+#endif
+
 /* arch/xen/i386/kernel/setup.c */
 extern start_info_t *xen_start_info;
 #ifdef CONFIG_XEN_PRIVILEGED_GUEST
Index: head-2006-11-06/include/asm-i386/mach-xen/asm/pgtable-2level-defs.h
===================================================================
--- head-2006-11-06.orig/include/asm-i386/mach-xen/asm/pgtable-2level-defs.h    
2006-09-21 09:57:46.000000000 +0200
+++ head-2006-11-06/include/asm-i386/mach-xen/asm/pgtable-2level-defs.h 
2006-11-10 16:09:20.000000000 +0100
@@ -9,7 +9,6 @@
 
 #define PGDIR_SHIFT    22
 #define PTRS_PER_PGD   1024
-#define PTRS_PER_PGD_NO_HV     (HYPERVISOR_VIRT_START >> PGDIR_SHIFT)
 
 /*
  * the i386 is two-level, so we don't really have any
Index: head-2006-11-06/include/asm-i386/mach-xen/asm/pgtable-3level-defs.h
===================================================================
--- head-2006-11-06.orig/include/asm-i386/mach-xen/asm/pgtable-3level-defs.h    
2006-09-21 09:57:46.000000000 +0200
+++ head-2006-11-06/include/asm-i386/mach-xen/asm/pgtable-3level-defs.h 
2006-11-10 16:09:25.000000000 +0100
@@ -8,7 +8,6 @@
  */
 #define PGDIR_SHIFT    30
 #define PTRS_PER_PGD   4
-#define PTRS_PER_PGD_NO_HV 4
 
 /*
  * PMD_SHIFT determines the size of the area a middle-level
Index: ./../xen/include/public/elfnote.h
===================================================================
--- ./../xen/include/public/elfnote.h   2006-08-25 15:36:10.000000000 +0200
+++ ./../xen/include/public/elfnote.h   2006-11-10 16:27:45.000000000 +0100
@@ -138,6 +138,15 @@
  */
 #define XEN_ELFNOTE_BSD_SYMTAB    11
 
+/*
+ * The lowest address the hypervisor hole can begin at (numeric).
+ *
+ * This must not be set higher than HYPERVISOR_VIRT_START. Its presence
+ * also indicates to the hypervisor that the kernel can deal with the
+ * hole starting at a higher address.
+ */
+#define XEN_ELFNOTE_HV_START_LOW  12
+
 #endif /* __XEN_PUBLIC_ELFNOTE_H__ */
 
 /*


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