[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


  • To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 27 May 2020 14:06:07 +0100
  • Authentication-results: esa4.hc3370-68.iphmx.com; dkim=none (message not signed) header.i=none
  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Wed, 27 May 2020 13:06:43 +0000
  • Ironport-sdr: 3bVAu9sXs27VQ/uE6l96fuNeL8FVmemCrw9I2Aj1uF3PqkLV095CwOOqqTkG7fDUWnvb0ACZEM XuGJaPOGspcFB/dmf4dUAG4qkJXSLICYTbVf+mByAjlE8X/qz9541EoyhQcRLJAne8869trGqu oh8pr4varGcZFxKoTmcREJ6jrleEKxQHSaBXcLcXQWIzf32alfAofYDUx8ofNh3sHXrF077hh3 LITD8ha08ElXDQRg1admQv4F5A2edbUN9/5Kj8YKnsDgwM8u+Wo1JKwYNEBIXWggZkvobX5qPX ZAk=
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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

This wants backporting a fairly long way, technically to Xen 4.6.
---
 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 3e0d9cbe98..a78b796fe5 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -703,11 +703,12 @@ 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;
 
@@ -720,30 +721,26 @@ 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. */
 
-- 
2.11.0




 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.