hw/pci: Basic support for PCI power management

The memory and IO BARs for devices are only accessible in the D0 power
state.  In other power states the PCI spec defines that the device
responds to TLPs and messages with an Unsupported Request response.

To approximate this behavior, consider the BARs as unmapped when the
device is not in the D0 power state.  This makes the BARs inaccessible
and has the additional bonus for vfio-pci that we don't attempt to DMA
map BARs for devices in a non-D0 power state.

To support this, an interface is added for devices to register the PM
capability, which allows central tracking to enforce valid transitions
and unmap BARs in non-D0 states.

NB. We currently have device models (eepro100 and pcie_pci_bridge)
that register a PM capability but do not set wmask to enable writes to
the power state field.  In order to maintain migration compatibility,
this new helper does not manage the wmask to enable guest writes to
initiate a power state change.  The contents and write access of the
PM capability are still managed by the caller.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250225215237.3314011-2-alex.williamson@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/pci/pci.c

@@ -435,6 +435,84 @@ static void pci_msi_trigger(PCIDevice *dev, MSIMessage msg)
                          attrs, NULL);
 }
 
+/*
+ * Register and track a PM capability.  If wmask is also enabled for the power
+ * state field of the pmcsr register, guest writes may change the device PM
+ * state.  BAR access is only enabled while the device is in the D0 state.
+ * Return the capability offset or negative error code.
+ */
+int pci_pm_init(PCIDevice *d, uint8_t offset, Error **errp)
+{
+    int cap = pci_add_capability(d, PCI_CAP_ID_PM, offset, PCI_PM_SIZEOF, errp);
+
+    if (cap < 0) {
+        return cap;
+    }
+
+    d->pm_cap = cap;
+    d->cap_present |= QEMU_PCI_CAP_PM;
+
+    return cap;
+}
+
+static uint8_t pci_pm_state(PCIDevice *d)
+{
+    uint16_t pmcsr;
+
+    if (!(d->cap_present & QEMU_PCI_CAP_PM)) {
+        return 0;
+    }
+
+    pmcsr = pci_get_word(d->config + d->pm_cap + PCI_PM_CTRL);
+
+    return pmcsr & PCI_PM_CTRL_STATE_MASK;
+}
+
+/*
+ * Update the PM capability state based on the new value stored in config
+ * space respective to the old, pre-write state provided.  If the new value
+ * is rejected (unsupported or invalid transition) restore the old value.
+ * Return the resulting PM state.
+ */
+static uint8_t pci_pm_update(PCIDevice *d, uint32_t addr, int l, uint8_t old)
+{
+    uint16_t pmc;
+    uint8_t new;
+
+    if (!(d->cap_present & QEMU_PCI_CAP_PM) ||
+        !range_covers_byte(addr, l, d->pm_cap + PCI_PM_CTRL)) {
+        return old;
+    }
+
+    new = pci_pm_state(d);
+    if (new == old) {
+        return old;
+    }
+
+    pmc = pci_get_word(d->config + d->pm_cap + PCI_PM_PMC);
+
+    /*
+     * Transitions to D1 & D2 are only allowed if supported.  Devices may
+     * only transition to higher D-states or to D0.
+     */
+    if ((!(pmc & PCI_PM_CAP_D1) && new == 1) ||
+        (!(pmc & PCI_PM_CAP_D2) && new == 2) ||
+        (old && new && new < old)) {
+        pci_word_test_and_clear_mask(d->config + d->pm_cap + PCI_PM_CTRL,
+                                     PCI_PM_CTRL_STATE_MASK);
+        pci_word_test_and_set_mask(d->config + d->pm_cap + PCI_PM_CTRL,
+                                   old);
+        trace_pci_pm_bad_transition(d->name, pci_dev_bus_num(d),
+                                    PCI_SLOT(d->devfn), PCI_FUNC(d->devfn),
+                                    old, new);
+        return old;
+    }
+
+    trace_pci_pm_transition(d->name, pci_dev_bus_num(d), PCI_SLOT(d->devfn),
+                            PCI_FUNC(d->devfn), old, new);
+    return new;
+}
+
 static void pci_reset_regions(PCIDevice *dev)
 {
     int r;
@@ -474,6 +552,11 @@ static void pci_do_device_reset(PCIDevice *dev)
                               pci_get_word(dev->wmask + PCI_INTERRUPT_LINE) |
                               pci_get_word(dev->w1cmask + PCI_INTERRUPT_LINE));
     dev->config[PCI_CACHE_LINE_SIZE] = 0x0;
+    /* Default PM state is D0 */
+    if (dev->cap_present & QEMU_PCI_CAP_PM) {
+        pci_word_test_and_clear_mask(dev->config + dev->pm_cap + PCI_PM_CTRL,
+                                     PCI_PM_CTRL_STATE_MASK);
+    }
     pci_reset_regions(dev);
     pci_update_mappings(dev);
 
@@ -1606,7 +1689,7 @@ static void pci_update_mappings(PCIDevice *d)
             continue;
 
         new_addr = pci_bar_address(d, i, r->type, r->size);
-        if (!d->enabled) {
+        if (!d->enabled || pci_pm_state(d)) {
             new_addr = PCI_BAR_UNMAPPED;
         }
 
@@ -1672,6 +1755,7 @@ uint32_t pci_default_read_config(PCIDevice *d,
 
 void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val_in, int l)
 {
+    uint8_t new_pm_state, old_pm_state = pci_pm_state(d);
     int i, was_irq_disabled = pci_irq_disabled(d);
     uint32_t val = val_in;
 
@@ -1684,11 +1768,16 @@ void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val_in, int
         d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
         d->config[addr + i] &= ~(val & w1cmask); /* W1C: Write 1 to Clear */
     }
+
+    new_pm_state = pci_pm_update(d, addr, l, old_pm_state);
+
     if (ranges_overlap(addr, l, PCI_BASE_ADDRESS_0, 24) ||
         ranges_overlap(addr, l, PCI_ROM_ADDRESS, 4) ||
         ranges_overlap(addr, l, PCI_ROM_ADDRESS1, 4) ||
-        range_covers_byte(addr, l, PCI_COMMAND))
+        range_covers_byte(addr, l, PCI_COMMAND) ||
+        !!new_pm_state != !!old_pm_state) {
         pci_update_mappings(d);
+    }
 
     if (ranges_overlap(addr, l, PCI_COMMAND, 2)) {
         pci_update_irq_disabled(d, was_irq_disabled);

hw/pci/trace-events

@@ -1,6 +1,8 @@
 # See docs/devel/tracing.rst for syntax documentation.
 
 # pci.c
+pci_pm_bad_transition(const char *dev, uint32_t bus, uint32_t slot, uint32_t func, uint8_t old, uint8_t new) "%s %02x:%02x.%x REJECTED PM transition D%d->D%d"
+pci_pm_transition(const char *dev, uint32_t bus, uint32_t slot, uint32_t func, uint8_t old, uint8_t new) "%s %02x:%02x.%x PM transition D%d->D%d"
 pci_update_mappings_del(const char *dev, uint32_t bus, uint32_t slot, uint32_t func, int bar, uint64_t addr, uint64_t size) "%s %02x:%02x.%x %d,0x%"PRIx64"+0x%"PRIx64
 pci_update_mappings_add(const char *dev, uint32_t bus, uint32_t slot, uint32_t func, int bar, uint64_t addr, uint64_t size) "%s %02x:%02x.%x %d,0x%"PRIx64"+0x%"PRIx64
 pci_route_irq(int dev_irq, const char *dev_path, int parent_irq, const char *parent_path) "IRQ %d @%s -> IRQ %d @%s"