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From:
ssnf <ssnf@ssnf.xyz>
Subject:
[PATCH 2/6] vmd: emulate i8253 square-wave counter correctly
To:
tech@openbsd.org
Cc:
ssnf <ssnf@ssnf.xyz>
Date:
Sat, 22 Aug 2026 19:02:43 +0000

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In mode 3, the visible PIT counter decrements by two per input clock.
vmd currently subtratcs one, causing guests to measure the TSC at
approximately twice its actual frequency.

Linux/KVM does the same 2*d treatment for mode 3.
---
 usr.sbin/vmd/i8253.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/usr.sbin/vmd/i8253.c b/usr.sbin/vmd/i8253.c
index 2e6c605b12d..decc8f8543d 100644
--- a/usr.sbin/vmd/i8253.c
+++ b/usr.sbin/vmd/i8253.c
@@ -138,6 +138,8 @@ i8253_do_readback(uint32_t data)
 				timespecsub(&now, &i8253_channel[i].ts, &delta);
 				ns = delta.tv_sec * 1000000000 + delta.tv_nsec;
 				ticks = ns / NS_PER_TICK;
+				if (i8253_channel[i].mode == TIMER_SQWAVE)
+					ticks *= 2;
 				if (i8253_channel[i].start)
 					i8253_channel[i].olatch =
 					    i8253_channel[i].start -
@@ -256,6 +258,8 @@ vcpu_exit_i8253(struct vm_run_params *vrp)
 				    &delta);
 				ns = delta.tv_sec * 1000000000 + delta.tv_nsec;
 				ticks = ns / NS_PER_TICK;
+				if (i8253_channel[sel].mode == TIMER_SQWAVE)
+					ticks *= 2;
 				if (i8253_channel[sel].start) {
 					i8253_channel[sel].olatch =
 					    i8253_channel[sel].start -
-- 
2.51.0