diff --git a/sys/dev/acpica/acpi_hpet.c b/sys/dev/acpica/acpi_hpet.c
index 2fa7122173c..5991c157e0d 100644
--- a/sys/dev/acpica/acpi_hpet.c
+++ b/sys/dev/acpica/acpi_hpet.c
@@ -59,6 +59,7 @@ __FBSDID("$FreeBSD$");
 #define HPET_VENDID_AMD		0x4353
 #define HPET_VENDID_INTEL	0x8086
 #define HPET_VENDID_NVIDIA	0x10de
+#define HPET_VENDID_SW		0x1166
 
 ACPI_SERIAL_DECL(hpet, "ACPI HPET support");
 
@@ -512,6 +513,13 @@ hpet_attach(device_t dev)
 	 */
 	if (vendor == HPET_VENDID_NVIDIA && rev <= 0x01)
 		sc->allowed_irqs = 0x00000000;
+	/*
+	 * ServerWorks HT1000 reported to have problems with IRQs >= 16.
+	 * Lower IRQs are working, but allowed mask is not set correctly.
+	 * Legacy_route mode works fine.
+	 */
+	if (vendor == HPET_VENDID_SW && rev <= 0x01)
+		sc->allowed_irqs = 0x00000000;
 	/*
 	 * Neither QEMU nor VirtualBox report supported IRQs correctly.
 	 * The only way to use HPET there is to specify IRQs manually