untrusted comment: verify with openbsd-68-base.pub
RWQZj25CSG5R2g9zYicLsPqttgR/D9AT3xvMoXLMjg+M0xwjaXI64knuaMJi985edBofoYOvlvl2b0Jrn8JLglfSssn6vyZAtgU=

OpenBSD 6.8 errata 023, June 8, 2021:

Recent Intel or any AMD machines could crash as the kernel did not
flush all TLBs that multi threaded processes require.

Apply by doing:
    signify -Vep /etc/signify/openbsd-68-base.pub -x 023_cpuswitch.patch.sig \
        -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
    KK=`sysctl -n kern.osversion | cut -d# -f1`
    cd /usr/src/sys/arch/`machine`/compile/$KK
    make obj
    make config
    make
    make install

Index: sys/arch/amd64/amd64/locore.S
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/locore.S,v
diff -u -p -r1.120 locore.S
--- sys/arch/amd64/amd64/locore.S	17 May 2020 13:48:30 -0000	1.120
+++ sys/arch/amd64/amd64/locore.S	2 Jun 2021 18:11:51 -0000
@@ -339,6 +339,7 @@ ENTRY(cpu_switchto)
 	shrq	$32,%rdx
 
 	/* If old proc exited, don't bother. */
+	xorl	%ecx,%ecx
 	testq	%r13,%r13
 	jz	switch_exited
 
@@ -346,18 +347,19 @@ ENTRY(cpu_switchto)
 	 * Save old context.
 	 *
 	 * Registers:
-	 *   %rax, %rcx - scratch
+	 *   %rax - scratch
 	 *   %r13 - old proc, then old pcb
+	 *   %rcx - old pmap if not P_SYSTEM
 	 *   %r12 - new proc
 	 *   %r9d - cpuid
 	 */
 
+	/* remember the pmap if not P_SYSTEM */
+	testl	$P_SYSTEM,P_FLAG(%r13)
 	movq	P_ADDR(%r13),%r13
-
-	/* clear the old pmap's bit for the cpu */
+	jnz	0f
 	movq	PCB_PMAP(%r13),%rcx
-	lock
-	btrq	%r9,PM_CPUS(%rcx)
+0:
 
 	/* Save stack pointers. */
 	movq	%rsp,PCB_RSP(%r13)
@@ -413,14 +415,24 @@ restore_saved:
 	 * Restore saved context.
 	 *
 	 * Registers:
-	 *   %rax, %rcx, %rdx - scratch
-	 *   %r13 - new pcb
+	 *   %rax, %rdx - scratch
+	 *   %rcx - old pmap if not P_SYSTEM
 	 *   %r12 - new process
+	 *   %r13 - new pcb
+	 *   %rbx - new pmap if not P_SYSTEM
 	 */
 
+	movq	P_ADDR(%r12),%r13
+
+	/* remember the pmap if not P_SYSTEM */
+	xorl	%ebx,%ebx
+	testl	$P_SYSTEM,P_FLAG(%r12)
+	jnz	1f
+	movq	PCB_PMAP(%r13),%rbx
+1:
+
 	/* No interrupts while loading new state. */
 	cli
-	movq	P_ADDR(%r12),%r13
 
 	/* Restore stack pointers. */
 	movq	PCB_RSP(%r13),%rsp
@@ -435,8 +447,25 @@ restore_saved:
 	movq	PCB_CR3(%r13),%rax	/* flags from cmpq unchanged */
 	jz	.Lsame_cr3
 
+	/* set the new pmap's bit for the cpu */
+	testq	%rbx,%rbx
+	jz	.Lno_new_pmap
+	lock
+	btsq	%r9,PM_CPUS(%rbx)
+#ifdef DIAGNOSTIC
+	jc	_C_LABEL(switch_pmcpu_set)
+#endif
+.Lno_new_pmap:
+
 	movq	%rax,%cr3			/* %rax used below too */
 
+	/* clear the old pmap's bit for the cpu */
+	testq	%rcx,%rcx
+	jz	.Lno_old_pmap
+	lock
+	btrq	%r9,PM_CPUS(%rcx)
+.Lno_old_pmap:
+
 .Lsame_cr3:
 	/*
 	 * If we switched from a userland thread with a shallow call stack
@@ -448,14 +477,13 @@ restore_saved:
 	RET_STACK_REFILL_WITH_RCX
 
 	/* Don't bother with the rest if switching to a system process. */
-	testl	$P_SYSTEM,P_FLAG(%r12)
-	jnz	switch_restored
+	testq	%rbx,%rbx
+	jz	switch_restored
 
 	/* record the bits needed for future U-->K transition */
 	movq	PCB_KSTACK(%r13),%rdx
 	subq	$FRAMESIZE,%rdx
 	movq	%rdx,CPUVAR(KERN_RSP)
-	movq	PCB_PMAP(%r13),%rcx
 
 	CODEPATCH_START
 	/*
@@ -463,19 +491,12 @@ restore_saved:
 	 * then record them in cpu_info for easy access in syscall and
 	 * interrupt trampolines.
 	 */
-	movq	PM_PDIRPA_INTEL(%rcx),%rdx
+	movq	PM_PDIRPA_INTEL(%rbx),%rdx
 	orq	cr3_reuse_pcid,%rax
 	orq	cr3_pcid_proc_intel,%rdx
 	movq	%rax,CPUVAR(KERN_CR3)
 	movq	%rdx,CPUVAR(USER_CR3)
 	CODEPATCH_END(CPTAG_MELTDOWN_NOP)
-
-	/* set the new pmap's bit for the cpu */
-	lock
-	btsq	%r9,PM_CPUS(%rcx)
-#ifdef DIAGNOSTIC
-	jc	_C_LABEL(switch_pmcpu_set)
-#endif
 
 switch_restored:
 	SET_CURPCB(%r13)
