[Xen-devel] [PATCH v2] x86/boot: Rationalise stack handling during early boot

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v2:
 * Use 'i' rather than 'ir' for [cs]
 * Adjust stack_base[0] calculation in smp_prepare_cpus()
---
 xen/arch/x86/boot/head.S    | 10 +++++++---
 xen/arch/x86/boot/x86_64.S  |  3 +--
 xen/arch/x86/efi/efi-boot.h | 15 ++++++++-------
 xen/arch/x86/smpboot.c      |  4 ++--
 4 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index c730810461..250587fdf0 100644
--- a/xen/arch/x86/boot/head.S
+++ b/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
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index b54d3aceea..0acf5e860c 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/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
diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index 676d616ff8..9c036d5f4c 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/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();
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 301f746979..c9d1ab4423 100644
--- a/xen/arch/x86/smpboot.c
+++ b/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 )
-- 
2.11.0


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