From 110c477c2ede4d06d3a95a2bfda5c4c58d2af516 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Fri, 21 Jun 2019 02:38:41 -0400 Subject: [PATCH] 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 Reviewed-by: Marcel Apfelbaum Reviewed-by: Igor Mammedov Tested-by: Igor Mammedov --- hw/pci/pcie.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) 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); }