pci: SLT must be RO

current code sets PCI_SEC_LATENCY_TIMER to RW, but for
pcie to pcie bridges it must be RO 0 according to
pci express spec which says:
    This register does not apply to PCI Express. It must be read-only
    and hardwired to 00h. For PCI Express to PCI/PCI-X Bridges, refer to the
    [PCIe-to-PCI-PCI-X-Bridge] for requirements for this register.

also, fix typo in comment where it's made writeable - this typo
is likely what prevented us noticing we violate this requirement
in the 1st place.

Reported-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-Id: <de9d05366a70172e1789d10591dbe59e39c3849c.1693432039.git.mst@redhat.com>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

4 files changed, 22 insertions, 2 deletions

diff --git a/hw/core/machine.c b/hw/core/machine.c
index cb38b8cf4c..9ae8f793ae 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -32,6 +32,7 @@
 #include "qemu/error-report.h"
 #include "sysemu/qtest.h"
 #include "hw/pci/pci.h"
+#include "hw/pci/pci_bridge.h"
 #include "hw/mem/nvdimm.h"
 #include "migration/global_state.h"
 #include "migration/vmstate.h"
@@ -40,7 +41,9 @@
 #include "hw/virtio/virtio-pci.h"
 #include "hw/virtio/virtio-net.h"
 
-GlobalProperty hw_compat_8_1[] = {};
+GlobalProperty hw_compat_8_1[] = {
+    { TYPE_PCI_BRIDGE, "x-pci-express-writeable-slt-bug", "true" },
+};
 const size_t hw_compat_8_1_len = G_N_ELEMENTS(hw_compat_8_1);
 
 GlobalProperty hw_compat_8_0[] = {
diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index 881d774fb6..b0d21bf43a 100644
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -893,7 +893,7 @@ static void pci_init_w1cmask(PCIDevice *dev)
 static void pci_init_mask_bridge(PCIDevice *d)
 {
     /* PCI_PRIMARY_BUS, PCI_SECONDARY_BUS, PCI_SUBORDINATE_BUS and
-       PCI_SEC_LETENCY_TIMER */
+       PCI_SEC_LATENCY_TIMER */
     memset(d->wmask + PCI_PRIMARY_BUS, 0xff, 4);
 
     /* base and limit */
diff --git a/hw/pci/pci_bridge.c b/hw/pci/pci_bridge.c
index e7b9345615..6a4e38856d 100644
--- a/hw/pci/pci_bridge.c
+++ b/hw/pci/pci_bridge.c
@@ -38,6 +38,7 @@
 #include "qapi/error.h"
 #include "hw/acpi/acpi_aml_interface.h"
 #include "hw/acpi/pci.h"
+#include "hw/qdev-properties.h"
 
 /* PCI bridge subsystem vendor ID helper functions */
 #define PCI_SSVID_SIZEOF        8
@@ -385,6 +386,11 @@ void pci_bridge_initfn(PCIDevice *dev, const char *typename)
     pci_bridge_region_init(br);
     QLIST_INIT(&sec_bus->child);
     QLIST_INSERT_HEAD(&parent->child, sec_bus, sibling);
+
+    /* For express secondary buses, secondary latency timer is RO 0 */
+    if (pci_bus_is_express(sec_bus) && !br->pcie_writeable_slt_bug) {
+        dev->wmask[PCI_SEC_LATENCY_TIMER] = 0;
+    }
 }
 
 /* default qdev clean up function for PCI-to-PCI bridge */
@@ -466,10 +472,18 @@ int pci_bridge_qemu_reserve_cap_init(PCIDevice *dev, int cap_offset,
     return 0;
 }
 
+static Property pci_bridge_properties[] = {
+    DEFINE_PROP_BOOL("x-pci-express-writeable-slt-bug", PCIBridge,
+                     pcie_writeable_slt_bug, false),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
 static void pci_bridge_class_init(ObjectClass *klass, void *data)
 {
     AcpiDevAmlIfClass *adevc = ACPI_DEV_AML_IF_CLASS(klass);
+    DeviceClass *k = DEVICE_CLASS(klass);
 
+    device_class_set_props(k, pci_bridge_properties);
     adevc->build_dev_aml = build_pci_bridge_aml;
 }
 
diff --git a/include/hw/pci/pci_bridge.h b/include/hw/pci/pci_bridge.h
index ea54a81a15..5cd452115a 100644
--- a/include/hw/pci/pci_bridge.h
+++ b/include/hw/pci/pci_bridge.h
@@ -77,6 +77,9 @@ struct PCIBridge {
 
     pci_map_irq_fn map_irq;
     const char *bus_name;
+
+    /* SLT is RO for PCIE to PCIE bridges, but old QEMU versions had it RW */
+    bool pcie_writeable_slt_bug;
 };
 
 #define PCI_BRIDGE_DEV_PROP_CHASSIS_NR "chassis_nr"