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authorMichael S. Tsirkin <mst@redhat.com>2019-06-21 02:38:41 -0400
committerMichael S. Tsirkin <mst@redhat.com>2019-07-01 09:17:30 -0400
commit110c477c2ede4d06d3a95a2bfda5c4c58d2af516 (patch)
tree84288ea3130cd95cbc4d5eeff020081e3ab8fe39 /hw/pci
parent2841ab435bca9f102311e01bf157d5fa878935dc (diff)
pcie: work around for racy guest init
During boot, linux guests tend to clear all bits in pcie slot status register which is used for hotplug. If they clear bits that weren't set this is racy and will lose events: not a big problem for manual hotplug on bare-metal, but a problem for us. For example, the following is broken ATM: /x86_64-softmmu/qemu-system-x86_64 -enable-kvm -S -machine q35 \ -device pcie-root-port,id=pcie_root_port_0,slot=2,chassis=2,addr=0x2,bus=pcie.0 \ -device virtio-balloon-pci,id=balloon,bus=pcie_root_port_0 \ -monitor stdio disk.qcow2 (qemu)device_del balloon (qemu)cont Balloon isn't deleted as it should. As a work-around, detect this attempt to clear slot status and revert status to what it was before the write. Note: in theory this can be detected as a duplicate button press which cancels the previous press. Does not seem to happen in practice as guests seem to only have this bug during init. Note2: the right thing to do is probably to fix Linux to read status before clearing it, and act on the bits that are set. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com> Reviewed-by: Igor Mammedov <imammedo@redhat.com> Tested-by: Igor Mammedov <imammedo@redhat.com>
Diffstat (limited to 'hw/pci')
-rw-r--r--hw/pci/pcie.c19
1 files changed, 19 insertions, 0 deletions
diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
index f8490a00de..c605d32dd4 100644
--- a/hw/pci/pcie.c
+++ b/hw/pci/pcie.c
@@ -610,6 +610,25 @@ void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slt_ctl, uint16_t slt_s
uint16_t sltsta = pci_get_word(exp_cap + PCI_EXP_SLTSTA);
if (ranges_overlap(addr, len, pos + PCI_EXP_SLTSTA, 2)) {
+ /*
+ * Guests tend to clears all bits during init.
+ * If they clear bits that weren't set this is racy and will lose events:
+ * not a big problem for manual button presses, but a problem for us.
+ * As a work-around, detect this and revert status to what it was
+ * before the write.
+ *
+ * Note: in theory this can be detected as a duplicate button press
+ * which cancels the previous press. Does not seem to happen in
+ * practice as guests seem to only have this bug during init.
+ */
+#define PCIE_SLOT_EVENTS (PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD | \
+ 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);
+ pci_set_word(exp_cap + PCI_EXP_SLTSTA, sltsta);
+ }
hotplug_event_clear(dev);
}