pcie: minor cleanups for slot control/status

Rename function arguments to make intent clearer.
Better documentation for slot control logic.

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>

hw/pci/pcie.c

@@ -602,7 +602,8 @@ void pcie_cap_slot_get(PCIDevice *dev, uint16_t *slt_ctl, uint16_t *slt_sta)
     *slt_sta = pci_get_word(exp_cap + PCI_EXP_SLTSTA);
 }
 
-void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slt_ctl, uint16_t slt_sta,
+void pcie_cap_slot_write_config(PCIDevice *dev,
+                                uint16_t old_slt_ctl, uint16_t old_slt_sta,
                                 uint32_t addr, uint32_t val, int len)
 {
     uint32_t pos = dev->exp.exp_cap;
@@ -625,8 +626,8 @@ void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slt_ctl, uint16_t slt_s
                           PCI_EXP_SLTSTA_MRLSC | PCI_EXP_SLTSTA_PDC | \
                           PCI_EXP_SLTSTA_CC)
 
-        if (val & ~slt_sta & PCIE_SLOT_EVENTS) {
-            sltsta = (sltsta & ~PCIE_SLOT_EVENTS) | (slt_sta & PCIE_SLOT_EVENTS);
+        if (val & ~old_slt_sta & PCIE_SLOT_EVENTS) {
+            sltsta = (sltsta & ~PCIE_SLOT_EVENTS) | (old_slt_sta & PCIE_SLOT_EVENTS);
             pci_set_word(exp_cap + PCI_EXP_SLTSTA, sltsta);
         }
         hotplug_event_clear(dev);
@@ -646,13 +647,17 @@ void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slt_ctl, uint16_t slt_s
     }
 
     /*
-     * If the slot is polulated, power indicator is off and power
+     * If the slot is populated, power indicator is off and power
      * controller is off, it is safe to detach the devices.
+     *
+     * Note: don't detach if condition was already true:
+     * this is a work around for guests that overwrite
+     * control of powered off slots before powering them on.
      */
     if ((sltsta & PCI_EXP_SLTSTA_PDS) && (val & PCI_EXP_SLTCTL_PCC) &&
         (val & PCI_EXP_SLTCTL_PIC_OFF) == PCI_EXP_SLTCTL_PIC_OFF &&
-        (!(slt_ctl & PCI_EXP_SLTCTL_PCC) ||
-        (slt_ctl & PCI_EXP_SLTCTL_PIC_OFF) != PCI_EXP_SLTCTL_PIC_OFF)) {
+        (!(old_slt_ctl & PCI_EXP_SLTCTL_PCC) ||
+        (old_slt_ctl & PCI_EXP_SLTCTL_PIC_OFF) != PCI_EXP_SLTCTL_PIC_OFF)) {
         PCIBus *sec_bus = pci_bridge_get_sec_bus(PCI_BRIDGE(dev));
         pci_for_each_device(sec_bus, pci_bus_num(sec_bus),
                             pcie_unplug_device, NULL);

include/hw/pci/pcie.h

@@ -108,7 +108,8 @@ void pcie_cap_lnkctl_reset(PCIDevice *dev);
 void pcie_cap_slot_init(PCIDevice *dev, uint16_t slot);
 void pcie_cap_slot_reset(PCIDevice *dev);
 void pcie_cap_slot_get(PCIDevice *dev, uint16_t *slot_ctl, uint16_t *slt_sta);
-void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slot_ctl, uint16_t slt_sta,
+void pcie_cap_slot_write_config(PCIDevice *dev,
+                                uint16_t old_slot_ctl, uint16_t old_slt_sta,
                                 uint32_t addr, uint32_t val, int len);
 int pcie_cap_slot_post_load(void *opaque, int version_id);
 void pcie_cap_slot_push_attention_button(PCIDevice *dev);