Merge tag 'x86-core-2023-06-26' of ssh://gitolite.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 core updates from Thomas Gleixner:
 "A set of fixes for kexec(), reboot and shutdown issues:

   - Ensure that the WBINVD in stop_this_cpu() has been completed before
     the control CPU proceedes.

     stop_this_cpu() is used for kexec(), reboot and shutdown to park
     the APs in a HLT loop.

     The control CPU sends an IPI to the APs and waits for their CPU
     online bits to be cleared. Once they all are marked "offline" it
     proceeds.

     But stop_this_cpu() clears the CPU online bit before issuing
     WBINVD, which means there is no guarantee that the AP has reached
     the HLT loop.

     This was reported to cause intermittent reboot/shutdown failures
     due to some dubious interaction with the firmware.

     This is not only a problem of WBINVD. The code to actually "stop"
     the CPU which runs between clearing the online bit and reaching the
     HLT loop can cause large enough delays on its own (think
     virtualization). That's especially dangerous for kexec() as kexec()
     expects that all APs are in a safe state and not executing code
     while the boot CPU jumps to the new kernel. There are more issues
     vs kexec() which are addressed separately.

     Cure this by implementing an explicit synchronization point right
     before the AP reaches HLT. This guarantees that the AP has
     completed the full stop proceedure.

   - Fix the condition for WBINVD in stop_this_cpu().

     The WBINVD in stop_this_cpu() is required for ensuring that when
     switching to or from memory encryption no dirty data is left in the
     cache lines which might cause a write back in the wrong more later.

     This checks CPUID directly because the feature bit might have been
     cleared due to a command line option.

     But that CPUID check accesses leaf 0x8000001f::EAX unconditionally.
     Intel CPUs return the content of the highest supported leaf when a
     non-existing leaf is read, while AMD CPUs return all zeros for
     unsupported leafs.

     So the result of the test on Intel CPUs is lottery and on AMD its
     just correct by chance.

     While harmless it's incorrect and causes the conditional wbinvd()
     to be issued where not required, which caused the above issue to be
     unearthed.

   - Make kexec() robust against AP code execution

     Ashok observed triple faults when doing kexec() on a system which
     had been booted with "nosmt".

     It turned out that the SMT siblings which had been brought up
     partially are parked in mwait_play_dead() to enable power savings.

     mwait_play_dead() is monitoring the thread flags of the AP's idle
     task, which has been chosen as it's unlikely to be written to.

     But kexec() can overwrite the previous kernel text and data
     including page tables etc. When it overwrites the cache lines
     monitored by an AP that AP resumes execution after the MWAIT on
     eventually overwritten text, stack and page tables, which obviously
     might end up in a triple fault easily.

     Make this more robust in several steps:

      1) Use an explicit per CPU cache line for monitoring.

      2) Write a command to these cache lines to kick APs out of MWAIT
         before proceeding with kexec(), shutdown or reboot.

         The APs confirm the wakeup by writing status back and then
         enter a HLT loop.

      3) If the system uses INIT/INIT/STARTUP for AP bringup, park the
         APs in INIT state.

         HLT is not a guarantee that an AP won't wake up and resume
         execution. HLT is woken up by NMI and SMI. SMI puts the CPU
         back into HLT (+/- firmware bugs), but NMI is delivered to the
         CPU which executes the NMI handler. Same issue as the MWAIT
         scenario described above.

         Sending an INIT/INIT sequence to the APs puts them into wait
         for STARTUP state, which is safe against NMI.

     There is still an issue remaining which can't be fixed: #MCE

     If the AP sits in HLT and receives a broadcast #MCE it will try to
     handle it with the obvious consequences.

     INIT/INIT clears CR4.MCE in the AP which will cause a broadcast
     #MCE to shut down the machine.

     So there is a choice between fire (HLT) and frying pan (INIT).
     Frying pan has been chosen as it's at least preventing the NMI
     issue.

     On systems which are not using INIT/INIT/STARTUP there is not much
     which can be done right now, but at least the obvious and easy to
     trigger MWAIT issue has been addressed"

* tag 'x86-core-2023-06-26' of ssh://gitolite.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/smp: Put CPUs into INIT on shutdown if possible
  x86/smp: Split sending INIT IPI out into a helper function
  x86/smp: Cure kexec() vs. mwait_play_dead() breakage
  x86/smp: Use dedicated cache-line for mwait_play_dead()
  x86/smp: Remove pointless wmb()s from native_stop_other_cpus()
  x86/smp: Dont access non-existing CPUID leaf
  x86/smp: Make stop_other_cpus() more robust

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