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/*
* This file and its contents are supplied under the terms of the
* Common Development and Distribution License ("CDDL"), version 1.0.
* You may only use this file in accordance with the terms of version
* 1.0 of the CDDL.
*
* A full copy of the text of the CDDL should have accompanied this
* source. A copy of the CDDL is also available via the Internet at
* http://www.illumos.org/license/CDDL.
*/
/*
* Copyright 2019 Joyent, Inc.
*/
#include <sys/asm_linkage.h>
/*
* This ASM file contains various routines that are designed to flush
* microarchitectural buffer state as part of dealing with the
* microarchitectural data sampling (MDS) vulnerabilities.
*
* These are called from various points in the system ranging from interrupts,
* before going idle, to returning from system calls. This means the following
* is true about the state of the system:
*
* o All register state is precious, we must not change register state upon
* entry or return from these functions.
*
* o %ds is valid.
*
* o %gs is arbitrary, it may be kernel or user. You cannot rely on it.
*
* o Interrupts should be disabled by the caller.
*
* o %cr3 is on the kernel-side and therefore we still have access to kernel
* text. In other words, we haven't switched back to the user page table.
*
* o It is up to the caller to insure that a sufficient serializing instruction
* has been executed after this to make sure any pending speculations are
* captured. In general, this should be handled by the fact that callers of
* this are either going to change privilege levels or halt, which makes
* these operations safer.
*/
/*
* By default, x86_md_clear is disabled until the system determines that
* it both needs MDS related mitigations and we have microcode that
* provides the needed functionality.
*
* The VERW instruction clobbers flags which is why it's important that
* we save and restore them here.
*/
ENTRY_NP(x86_md_clear)
ret
pushfq
subq $8, %rsp
mov %ds, (%rsp)
verw (%rsp)
addq $8, %rsp
popfq
ret
SET_SIZE(x86_md_clear)
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