Re: [Qemu-devel] [PATCH 3/3] pcie: work around for racy guest init

[Marcel Apfelbaum <marcel.apfelbaum@gmail.com>]

On 6/21/19 9:46 AM, Michael S. Tsirkin wrote:
> 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>
> ---
>   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);
>       }
>   

Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>

Thanks,
Marcel