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

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

On Mon, Jul 01, 2019 at 02:04:02PM +0200, Igor Mammedov wrote:
> On Mon, 1 Jul 2019 05:20:41 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 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.
> 
> I did notice it but still don't see where 
>   exp_cap + PCI_EXP_SLTSTA
> is being modified in call chain between
>   pcie_cap_slot_get(d, &slt_ctl, &slt_sta)
>   ...
>   pcie_cap_slot_write_config():
>       sltsta = pci_get_word(exp_cap + PCI_EXP_SLTSTA)

It's modified by pci_bridge_write_config which calls pci_default_write_config.


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