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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 25, 2019 at 03:07:30PM +0200, Igor Mammedov wrote:
> On Fri, 21 Jun 2019 02:46:50 -0400
> "Michael S. Tsirkin" <mst@redhat.com> 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);
> I'm reading it as:
>   sltsta = LOWER_PART(sltsta) | UPPER_PART(sltsta)
> which basically
>   sltsta = sltsta
> or am I missing something here?

You are missing the underscore.

slt_sta is the old value.
sltsta is the new value.

> > +            pci_set_word(exp_cap + PCI_EXP_SLTSTA, sltsta);
> > +        }
> >          hotplug_event_clear(dev);
> >      }
> >