1 /*
   2  * This file and its contents are supplied under the terms of the
   3  * Common Development and Distribution License ("CDDL"), version 1.0.
   4  * You may only use this file in accordance with the terms of version
   5  * 1.0 of the CDDL.
   6  *
   7  * A full copy of the text of the CDDL should have accompanied this
   8  * source.  A copy of the CDDL is also available via the Internet at
   9  * http://www.illumos.org/license/CDDL.
  10  */
  11 
  12 /*
  13  * Copyright 2019 Joyent, Inc.
  14  */
  15 
  16 #include <sys/asm_linkage.h>
  17 
  18 /*
  19  * This ASM file contains various routines that are designed to flush
  20  * microarchitectural buffer state as part of dealing with the
  21  * microarchitectural data sampling (MDS) vulnerabilities.
  22  *
  23  * These are called from various points in the system ranging from interrupts,
  24  * before going idle, to returning from system calls. This means the following
  25  * is true about the state of the system:
  26  *
  27  *  o All register state is precious, we must not change register state upon
  28  *    entry or return from these functions.
  29  *
  30  *  o %ds is valid.
  31  *
  32  *  o %gs is arbitrary, it may be kernel or user. You cannot rely on it.
  33  *
  34  *  o Interrupts should be disabled by the caller.
  35  *
  36  *  o %cr3 is on the kernel-side and therefore we still have access to kernel
  37  *    text. In other words, we haven't switched back to the user page table.
  38  *
  39  *  o It is up to the caller to insure that a sufficient serializing instruction
  40  *    has been executed after this to make sure any pending speculations are
  41  *    captured. In general, this should be handled by the fact that callers of
  42  *    this are either going to change privilege levels or halt, which makes
  43  *    these operations safer.
  44  */
  45 
  46         /*
  47          * By default, x86_md_clear is disabled until the system determines that
  48          * it both needs MDS related mitigations and we have microcode that
  49          * provides the needed functionality.
  50          *
  51          * The VERW instruction clobbers flags which is why it's important that
  52          * we save and restore them here.
  53          */
  54         ENTRY_NP(x86_md_clear)
  55         ret
  56         pushfq
  57         subq    $8, %rsp
  58         mov     %ds, (%rsp)
  59         verw    (%rsp)
  60         addq    $8, %rsp
  61         popfq
  62         ret
  63         SET_SIZE(x86_md_clear)