[xen staging] x86/crash: Drop manual hooking of exception_table[]

[patchbot@xxxxxxx]

commit e7f147bf4ac725492962a501da72f5ab6be682db
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Oct 7 14:02:10 2021 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Nov 29 13:53:05 2021 +0000

    x86/crash: Drop manual hooking of exception_table[]
    
    NMI hooking in the crash path has undergone several revisions since its
    introduction.  What we have now is not sufficiently different from the 
regular
    nmi_callback() mechanism to warrant special casing.
    
    Use set_nmi_callback() directly, and do away with patching a read-only data
    structure via a read-write alias.  This also means that the
    vmx_vmexit_handler() can and should call do_nmi() directly, rather than
    indirecting through the exception table to pick up the crash path hook.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/crash.c       | 15 ++-------------
 xen/arch/x86/hvm/vmx/vmx.c |  2 +-
 xen/arch/x86/traps.c       |  5 +++++
 3 files changed, 8 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
index 0611b4fb9b..f6264946a6 100644
--- a/xen/arch/x86/crash.c
+++ b/xen/arch/x86/crash.c
@@ -36,10 +36,8 @@ static unsigned int crashing_cpu;
 static DEFINE_PER_CPU_READ_MOSTLY(bool, crash_save_done);
 
 /* This becomes the NMI handler for non-crashing CPUs, when Xen is crashing. */
-static void noreturn do_nmi_crash(const struct cpu_user_regs *regs)
+static int noreturn do_nmi_crash(const struct cpu_user_regs *regs, int cpu)
 {
-    unsigned int cpu = smp_processor_id();
-
     stac();
 
     /* nmi_shootdown_cpus() should ensure that this assertion is correct. */
@@ -136,16 +134,7 @@ static void nmi_shootdown_cpus(void)
                     SYS_DESC_irq_gate, 0, &trap_nop);
     set_ist(&idt_tables[cpu][TRAP_machine_check], IST_NONE);
 
-    /*
-     * Ideally would be:
-     *   exception_table[TRAP_nmi] = &do_nmi_crash;
-     *
-     * but the exception_table is read only.  Access it via its directmap
-     * mappings.
-     */
-    write_atomic((unsigned long *)__va(__pa(&exception_table[TRAP_nmi])),
-                 (unsigned long)&do_nmi_crash);
-
+    set_nmi_callback(do_nmi_crash);
     smp_send_nmi_allbutself();
 
     msecs = 1000; /* Wait at most a second for the other cpus to stop */
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index d403e2d806..37c31c08b9 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3887,7 +3887,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
              ((intr_info & INTR_INFO_INTR_TYPE_MASK) ==
               MASK_INSR(X86_EVENTTYPE_NMI, INTR_INFO_INTR_TYPE_MASK)) )
         {
-            exception_table[TRAP_nmi](regs);
+            do_nmi(regs);
             enable_nmis();
         }
         break;
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 3bc4cd5e1e..6d7d88c3c4 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1883,6 +1883,11 @@ void do_nmi(const struct cpu_user_regs *regs)
     this_cpu(nmi_count)++;
     nmi_enter();
 
+    /*
+     * Think carefully before putting any logic before this point.
+     * nmi_callback() might be the crash quiesce...
+     */
+
     callback = ACCESS_ONCE(nmi_callback);
     if ( unlikely(callback) && callback(regs, cpu) )
         goto out;
--
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