[xen staging] x86/boot: Fix load_system_tables() to be NMI/#MC-safe

[patchbot@xxxxxxx]

commit 9f3e9139fa6c3d620eb08dff927518fc88200b8d
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed May 27 13:48:45 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed May 27 16:44:04 2020 +0100

    x86/boot: Fix load_system_tables() to be NMI/#MC-safe
    
    During boot, load_system_tables() is used in reinit_bsp_stack() to switch 
the
    virtual addresses used from their .data/.bss alias, to their directmap 
alias.
    
    The structure assignment is implemented as a memset() to zero first, then a
    copy-in of the new data.  This causes the NMI/#MC stack pointers to
    transiently become 0, at a point where we may have an NMI watchdog running.
    
    Rewrite the logic using a volatile tss pointer (equivalent to, but more
    readable than, using ACCESS_ONCE() for all writes).
    
    This does drop the zeroing side effect for holes in the structure, but the
    backing memory for the TSS is fully zeroed anyway, and architecturally, they
    are all reserved.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/cpu/common.c | 49 ++++++++++++++++++++++-------------------------
 1 file changed, 23 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index c413188d00..09b911b3ba 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -730,11 +730,12 @@ static cpumask_t cpu_initialized;
  */
 void load_system_tables(void)
 {
-       unsigned int cpu = smp_processor_id();
+       unsigned int i, cpu = smp_processor_id();
        unsigned long stack_bottom = get_stack_bottom(),
                stack_top = stack_bottom & ~(STACK_SIZE - 1);
 
-       struct tss64 *tss = &this_cpu(tss_page).tss;
+       /* The TSS may be live.  Disuade any clever optimisations. */
+       volatile struct tss64 *tss = &this_cpu(tss_page).tss;
        seg_desc_t *gdt =
                this_cpu(gdt) - FIRST_RESERVED_GDT_ENTRY;
 
@@ -747,30 +748,26 @@ void load_system_tables(void)
                .limit = (IDT_ENTRIES * sizeof(idt_entry_t)) - 1,
        };
 
-       *tss = (struct tss64){
-               /* Main stack for interrupts/exceptions. */
-               .rsp0 = stack_bottom,
-
-               /* Ring 1 and 2 stacks poisoned. */
-               .rsp1 = 0x8600111111111111ul,
-               .rsp2 = 0x8600111111111111ul,
-
-               /*
-                * MCE, NMI and Double Fault handlers get their own stacks.
-                * All others poisoned.
-                */
-               .ist = {
-                       [IST_MCE - 1] = stack_top + IST_MCE * PAGE_SIZE,
-                       [IST_DF  - 1] = stack_top + IST_DF  * PAGE_SIZE,
-                       [IST_NMI - 1] = stack_top + IST_NMI * PAGE_SIZE,
-                       [IST_DB  - 1] = stack_top + IST_DB  * PAGE_SIZE,
-
-                       [IST_MAX ... ARRAY_SIZE(tss->ist) - 1] =
-                               0x8600111111111111ul,
-               },
-
-               .bitmap = IOBMP_INVALID_OFFSET,
-       };
+       /*
+        * Set up the TSS.  Warning - may be live, and the NMI/#MC must remain
+        * valid on every instruction boundary.  (Note: these are all
+        * semantically ACCESS_ONCE() due to tss's volatile qualifier.)
+        *
+        * rsp0 refers to the primary stack.  #MC, #DF, NMI and #DB handlers
+        * each get their own stacks.  No IO Bitmap.
+        */
+       tss->rsp0 = stack_bottom;
+       tss->ist[IST_MCE - 1] = stack_top + IST_MCE * PAGE_SIZE;
+       tss->ist[IST_DF  - 1] = stack_top + IST_DF  * PAGE_SIZE;
+       tss->ist[IST_NMI - 1] = stack_top + IST_NMI * PAGE_SIZE;
+       tss->ist[IST_DB  - 1] = stack_top + IST_DB  * PAGE_SIZE;
+       tss->bitmap = IOBMP_INVALID_OFFSET;
+
+       /* All other stack pointers poisioned. */
+       for ( i = IST_MAX; i < ARRAY_SIZE(tss->ist); ++i )
+               tss->ist[i] = 0x8600111111111111ul;
+       tss->rsp1 = 0x8600111111111111ul;
+       tss->rsp2 = 0x8600111111111111ul;
 
        BUILD_BUG_ON(sizeof(*tss) <= 0x67); /* Mandated by the architecture. */
 
--
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