x86/kexec: Change NMI and MCE handling on kexec path

x86/kexec: Change NMI and MCE handling on kexec path

Experimentally, certain crash kernels will triple fault very early
after starting if started with NMIs disabled.  This was discovered
when experimenting with a debug keyhandler which deliberately created
a reentrant NMI, causing stack corruption.

Because of this discovered bug, and that the future changes to the NMI
handling will make the kexec path more fragile, take the time now to
bullet-proof the kexec behaviour to be safer in more circumstances.

This patch adds three new low level routines:
 * nmi_crash
    This is a special NMI handler for using during a kexec crash.
 * enable_nmis
    This function enables NMIs by executing an iret-to-self, to
    disengage the hardware NMI latch.
 * trap_nop
    This is a no op handler which irets immediately.  It is not
    declared
    with ENTRY() to avoid the extra alignment overhead.

And adds three new IDT entry helper routines:
 * _write_gate_lower
    This is a substitute for using cmpxchg16b to update a 128bit
    structure at once.  It assumes that the top 64 bits are unchanged
    (and ASSERT()s the fact) and performs a regular write on the lower
    64 bits.
 * _set_gate_lower
    This is functionally equivalent to the already present
    _set_gate(), except it uses _write_gate_lower rather than updating
    both 64bit values.
 * _update_gate_addr_lower
    This is designed to update an IDT entry handler only, without
    altering any other settings in the entry.  It also uses
    _write_gate_lower.

The IDT entry helpers are required because:
  * Is it unsafe to attempt a disable/update/re-enable cycle on the
    NMI or MCE IDT entries.
  * We need to be able to update NMI handlers without changing the IST
    entry.

As a result, the new behaviour of the kexec_crash path is:

nmi_shootdown_cpus() will:

 * Disable the crashing cpus NMI/MCE interrupt stack tables.
    Disabling the stack tables removes race conditions which would
    lead
    to corrupt exception frames and infinite loops.  As this pcpu is
    never planning to execute a sysret back to a pv vcpu, the update
    is
    safe from a security point of view.

 * Swap the NMI trap handlers.
    The crashing pcpu gets the nop handler, to prevent it getting
    stuck in
    an NMI context, causing a hang instead of crash.  The non-crashing
    pcpus all get the nmi_crash handler which is designed never to
    return.

do_nmi_crash() will:

 * Save the crash notes and shut the pcpu down.
    There is now an extra per-cpu variable to prevent us from
    executing this multiple times.  In the case where we reenter
    midway through, attempt the whole operation again in preference to
    not completing it in the first place.

 * Set up another NMI at the LAPIC.
    Even when the LAPIC has been disabled, the ID and command
    registers are still usable.  As a result, we can deliberately
    queue up a new NMI to re-interrupt us later if NMIs get unlatched.
    Because of the call to __stop_this_cpu(), we have to hand craft
    self_nmi() to be safe from General Protection Faults.

 * Fall into infinite loop.

machine_kexec() will:

  * Swap the MCE handlers to be a nop.
     We cannot prevent MCEs from being delivered when we pass off to
     the crash kernel, and the less Xen context is being touched the
     better.

  * Explicitly enable NMIs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>
Minor style changes.

Signed-off-by: Keir Fraser <keir@xen.org>
Committed-by: Keir Fraser <keir@xen.org>