Re: [Qemu-devel] [PATCH v2] pci-assign: Add MSI affinity support

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

On Mon, May 20, 2013 at 07:51:40AM -0500, Anthony Liguori wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Mon, May 13, 2013 at 02:20:02PM -0600, Alex Williamson wrote:
> >> To support guest MSI affinity changes update the MSI message any time
> >> the guest writes to the address or data fields.
> >> 
> >> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> >
> > Seems the only way we can fix this for 1.5.
> >
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> Is this a bug or a feature?  Can someone describe the scenario in which
> the bug occurs?
> 
> Regards,
> 
> Anthony Liguori

It's a bug. Here's how it occurs w.g. with a linux guest:

- guest kernel enables MSI, interrupts are sent to CPU0
- irqbalance runs in guest and moves MSI to CPU1
- guest kernel updates MSI register with new data

without this patch interrupts keep getting sent to CPU0
in violation of the spec.

> >
> >> ---
> >> 
> >> v2: Don't cache MSIMessage
> >> 
> >>  hw/i386/kvm/pci-assign.c |   18 ++++++++++++++++++
> >>  1 file changed, 18 insertions(+)
> >> 
> >> diff --git a/hw/i386/kvm/pci-assign.c b/hw/i386/kvm/pci-assign.c
> >> index c1e08ec..ff85590 100644
> >> --- a/hw/i386/kvm/pci-assign.c
> >> +++ b/hw/i386/kvm/pci-assign.c
> >> @@ -1026,6 +1026,21 @@ static void assigned_dev_update_msi(PCIDevice *pci_dev)
> >>      }
> >>  }
> >>  
> >> +static void assigned_dev_update_msi_msg(PCIDevice *pci_dev)
> >> +{
> >> +    AssignedDevice *assigned_dev = DO_UPCAST(AssignedDevice, dev, pci_dev);
> >> +    uint8_t ctrl_byte = pci_get_byte(pci_dev->config + pci_dev->msi_cap +
> >> +                                     PCI_MSI_FLAGS);
> >> +
> >> +    if (assigned_dev->assigned_irq_type != ASSIGNED_IRQ_MSI ||
> >> +        !(ctrl_byte & PCI_MSI_FLAGS_ENABLE)) {
> >> +        return;
> >> +    }
> >> +
> >> +    kvm_irqchip_update_msi_route(kvm_state, assigned_dev->msi_virq[0],
> >> +                                 msi_get_message(pci_dev, 0));
> >> +}
> >> +
> >>  static bool assigned_dev_msix_masked(MSIXTableEntry *entry)
> >>  {
> >>      return (entry->ctrl & cpu_to_le32(0x1)) != 0;
> >> @@ -1201,6 +1216,9 @@ static void assigned_dev_pci_write_config(PCIDevice *pci_dev, uint32_t address,
> >>          if (range_covers_byte(address, len,
> >>                                pci_dev->msi_cap + PCI_MSI_FLAGS)) {
> >>              assigned_dev_update_msi(pci_dev);
> >> +        } else if (ranges_overlap(address, len, /* 32bit MSI only */
> >> +                                  pci_dev->msi_cap + PCI_MSI_ADDRESS_LO, 6)) {
> >> +            assigned_dev_update_msi_msg(pci_dev);
> >>          }
> >>      }
> >>      if (assigned_dev->cap.available & ASSIGNED_DEVICE_CAP_MSIX) {