x86/boot: Rationalise stack handling during early boot

The top (numerically higher addresses) of cpu0_stack[] contains the BSP's
cpu_info block.  Logic in Xen expects this to be initialised to 0, but this
area of stack is also used during early boot.

Update the head.S code to avoid using the cpu_info block.  Additionally,
update the stack_start variable to match, which avoids __high_start() and
efi_arch_post_exit_boot() needing to make the adjustment manually.

Finally, leave a big warning by the BIOS BSS initialisation, because it is by
no means obvious that the stack doesn't survive the REP STOS.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/boot/head.S

@@ -400,7 +400,7 @@ __pvh_start:
         sub     $sym_offs(1b), %esi
 
         /* Set up stack. */
-        lea     STACK_SIZE + sym_esi(cpu0_stack), %esp
+        lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
 
         mov     %ebx, sym_esi(pvh_start_info_pa)
 
@@ -447,7 +447,7 @@ __start:
         sub     $sym_offs(1b), %esi
 
         /* Set up stack. */
-        lea     STACK_SIZE + sym_esi(cpu0_stack), %esp
+        lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
 
         /* Bootloaders may set multiboot{1,2}.mem_lower to a nonzero value. */
         xor     %edx,%edx
@@ -616,7 +616,11 @@ trampoline_setup:
         cmpb    $0,sym_fs(efi_platform)
         jnz     1f
 
-        /* Initialize BSS (no nasty surprises!). */
+        /*
+         * Initialise the BSS.
+         *
+         * !!! WARNING - also zeroes the current stack !!!
+         */
         lea     sym_esi(__bss_start), %edi
         lea     sym_esi(__bss_end), %ecx
         sub     %edi,%ecx

xen/arch/x86/boot/x86_64.S

@@ -16,7 +16,6 @@ ENTRY(__high_start)
         mov     %rcx,%cr4
 
         mov     stack_start(%rip),%rsp
-        or      $(STACK_SIZE-CPUINFO_sizeof),%rsp
 
         /* Reset EFLAGS (subsumes CLI and CLD). */
         pushq   $0
@@ -42,7 +41,7 @@ multiboot_ptr:
         .long   0
 
 GLOBAL(stack_start)
-        .quad   cpu0_stack
+        .quad   cpu0_stack + STACK_SIZE - CPUINFO_sizeof
 
         .section .data.page_aligned, "aw", @progbits
         .align PAGE_SIZE, 0

xen/arch/x86/efi/efi-boot.h

@@ -249,23 +249,24 @@ static void __init noreturn efi_arch_post_exit_boot(void)
                    "or     $"__stringify(X86_CR4_PGE)", %[cr4]\n\t"
                    "mov    %[cr4], %%cr4\n\t"
 #endif
-                   "movabs $__start_xen, %[rip]\n\t"
                    "lgdt   boot_gdtr(%%rip)\n\t"
-                   "mov    stack_start(%%rip), %%rsp\n\t"
                    "mov    %[ds], %%ss\n\t"
                    "mov    %[ds], %%ds\n\t"
                    "mov    %[ds], %%es\n\t"
                    "mov    %[ds], %%fs\n\t"
                    "mov    %[ds], %%gs\n\t"
-                   "movl   %[cs], 8(%%rsp)\n\t"
-                   "mov    %[rip], (%%rsp)\n\t"
-                   "lretq  %[stkoff]-16"
+
+                   /* Jump to higher mappings. */
+                   "mov    stack_start(%%rip), %%rsp\n\t"
+                   "movabs $__start_xen, %[rip]\n\t"
+                   "push   %[cs]\n\t"
+                   "push   %[rip]\n\t"
+                   "lretq"
                    : [rip] "=&r" (efer/* any dead 64-bit variable */),
                      [cr4] "+&r" (cr4)
                    : [cr3] "r" (idle_pg_table),
-                     [cs] "ir" (__HYPERVISOR_CS),
+                     [cs] "i" (__HYPERVISOR_CS),
                      [ds] "r" (__HYPERVISOR_DS),
-                     [stkoff] "i" (STACK_SIZE - sizeof(struct cpu_info)),
                      "D" (&mbi)
                    : "memory" );
     unreachable();

xen/arch/x86/smpboot.c

@@ -554,7 +554,7 @@ static int do_boot_cpu(int apicid, int cpu)
         printk("Booting processor %d/%d eip %lx\n",
                cpu, apicid, start_eip);
 
-    stack_start = stack_base[cpu];
+    stack_start = stack_base[cpu] + STACK_SIZE - sizeof(struct cpu_info);
 
     /* This grunge runs the startup process for the targeted processor. */
 
@@ -1084,7 +1084,7 @@ void __init smp_prepare_cpus(void)
     boot_cpu_physical_apicid = get_apic_id();
     x86_cpu_to_apicid[0] = boot_cpu_physical_apicid;
 
-    stack_base[0] = stack_start;
+    stack_base[0] = (void *)((unsigned long)stack_start & ~(STACK_SIZE - 1));
 
     rc = setup_cpu_root_pgt(0);
     if ( rc )