x86/xen: enable early use of set_fixmap in 32-bit Xen PV guest

Commit 7b25b9cb0d ("x86/xen/time: Initialize pv xen time in
init_hypervisor_platform()") moved the mapping of the shared info area
before pagetable_init(). This breaks booting as 32-bit PV guest as the
use of set_fixmap isn't possible at this time on 32-bit.

This can be worked around by populating the needed PMD on 32-bit
kernel earlier.

In order not to reimplement populate_extra_pte() using extend_brk()
for allocating new page tables extend alloc_low_pages() to do that in
case the early page table pool is not yet available.

Fixes: 7b25b9cb0d ("x86/xen/time: Initialize pv xen time in init_hypervisor_platform()")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/xen/enlighten_pv.c

@@ -122,6 +122,8 @@ static void __init xen_banner(void)
 
 static void __init xen_pv_init_platform(void)
 {
+	populate_extra_pte(fix_to_virt(FIX_PARAVIRT_BOOTMAP));
+
 	set_fixmap(FIX_PARAVIRT_BOOTMAP, xen_start_info->shared_info);
 	HYPERVISOR_shared_info = (void *)fix_to_virt(FIX_PARAVIRT_BOOTMAP);
 

arch/x86/xen/mmu_pv.c

@@ -2171,6 +2171,8 @@ void __init xen_relocate_p2m(void)
 #else	/* !CONFIG_X86_64 */
 static RESERVE_BRK_ARRAY(pmd_t, initial_kernel_pmd, PTRS_PER_PMD);
 static RESERVE_BRK_ARRAY(pmd_t, swapper_kernel_pmd, PTRS_PER_PMD);
+RESERVE_BRK(fixup_kernel_pmd, PAGE_SIZE);
+RESERVE_BRK(fixup_kernel_pte, PAGE_SIZE);
 
 static void __init xen_write_cr3_init(unsigned long cr3)
 {