Re: [Qemu-devel] [RFC v8.1 06/13] vfio: add check host bus reset is support or not

[Alex Williamson]

On Wed, 2015-06-03 at 08:52 +0800, Chen Fan wrote:
> On 06/03/2015 12:47 AM, Alex Williamson wrote:
> > On Tue, 2015-06-02 at 15:54 +0800, Chen Fan wrote:
> >> On 05/28/2015 05:32 AM, Alex Williamson wrote:
> >>> On Wed, 2015-05-27 at 10:46 +0800, Chen Fan wrote:
> >>>> we introduce a has_bus_reset capability to sign the vfio
> >>>> devices if support host bus reset.
> >>>>
> >>>> Signed-off-by: Chen Fan <address@hidden>
> >>>> ---
> >>>>    hw/vfio/pci.c | 123 
> >>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>>>    1 file changed, 123 insertions(+)
> >>>>
> >>>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >>>> index f4e7855..5934fd7 100644
> >>>> --- a/hw/vfio/pci.c
> >>>> +++ b/hw/vfio/pci.c
> >>>> @@ -33,6 +33,7 @@
> >>>>    #include "hw/pci/msix.h"
> >>>>    #include "hw/pci/pci.h"
> >>>>    #include "hw/pci/pci_bridge.h"
> >>>> +#include "hw/pci/pci_bus.h"
> >>>>    #include "qemu-common.h"
> >>>>    #include "qemu/error-report.h"
> >>>>    #include "qemu/event_notifier.h"
> >>>> @@ -170,6 +171,7 @@ typedef struct VFIOPCIDevice {
> >>>>        bool req_enabled;
> >>>>        bool has_flr;
> >>>>        bool has_pm_reset;
> >>>> +    bool has_bus_reset;
> >>> I still think that caching this is a bad idea, there's no point at which
> >>> we can blindly assume the capability is still present.
> >>>
> >>>>        bool rom_read_failed;
> >>>>    } VFIOPCIDevice;
> >>>>    
> >>>> @@ -203,6 +205,7 @@ static uint32_t vfio_pci_read_config(PCIDevice 
> >>>> *pdev, uint32_t addr, int len);
> >>>>    static void vfio_pci_write_config(PCIDevice *pdev, uint32_t addr,
> >>>>                                      uint32_t val, int len);
> >>>>    static void vfio_mmap_set_enabled(VFIOPCIDevice *vdev, bool enabled);
> >>>> +static void vfio_check_host_bus_reset(VFIOPCIDevice *vdev);
> >>>>    
> >>>>    /*
> >>>>     * Disabling BAR mmaping can be slow, but toggling it around INTx can
> >>>> @@ -2853,6 +2856,20 @@ static int vfio_setup_aer(VFIOPCIDevice *vdev, 
> >>>> int pos, uint16_t size)
> >>>>            dev_iter = pci_bridge_get_device(dev_iter->bus);
> >>>>        }
> >>>>    
> >>>> +    /*
> >>>> +     * Don't check bus reset capability when device is enabled during
> >>>> +     * qemu machine creation, which is done by machine init function.
> >>>> +     */
> >>>> +    if (DEVICE(vdev)->hotplugged) {
> >>>> +        vfio_check_host_bus_reset(vdev);
> >>>> +        if (!vdev->has_bus_reset) {
> >>>> +            error_report("vfio: Cannot enable AER for device %s, "
> >>>> +                         "which is not support host bus reset.",
> >>> "which does not support host bus reset."
> >>>
> >>>> +                         vdev->vbasedev.name);
> >>>> +            goto error;
> >>>> +        }
> >>>> +    }
> >>>> +
> >>>>        errcap = vfio_pci_read_config(pdev, pdev->exp.aer_cap + 
> >>>> PCI_ERR_CAP, 4);
> >>>>        /*
> >>>>         * The ability to record multiple headers is depending on
> >>>> @@ -3678,6 +3695,112 @@ static void vfio_setup_resetfn(VFIOPCIDevice 
> >>>> *vdev)
> >>>>        }
> >>>>    }
> >>>>    
> >>>> +struct VfioDeviceFind {
> >>> We use VFIOFooBar for all other camel case definitions, much like PCIBus
> >>> and PCIDevice below.
> >>>
> >>>> +    PCIBus *pbus;
> >>>> +    PCIDevice *pdev;
> >>>> +    bool found;
> >>>> +};
> >>>> +
> >>>> +static void find_devices(PCIBus *bus, void *opaque)
> >>>> +{
> >>>> +    struct VfioDeviceFind *find = opaque;
> >>>> +    int i;
> >>>> +
> >>>> +    if (find->found == true) {
> >>> if (find->found) {...
> >>>
> >>>> +        return;
> >>>> +    }
> >>>> +
> >>>> +    for (i = 0; i < ARRAY_SIZE(bus->devices); i++) {
> >>>> +        if (!bus->devices[i]) {
> >>>> +            continue;
> >>>> +        }
> >>>> +
> >>>> +        if (bus->devices[i] == find->pdev) {
> >>>> +            find->pbus = bus;
> >>>> +            find->found = true;
> >>>> +            break;
> >>>> +        }
> >>>> +    }
> >>>> +}
> >>>> +
> >>>> +static void vfio_check_host_bus_reset(VFIOPCIDevice *vdev)
> >>>> +{
> >>>> +    PCIBus *bus = vdev->pdev.bus;
> >>>> +    struct vfio_pci_hot_reset_info *info = NULL;
> >>>> +    struct vfio_pci_dependent_device *devices;
> >>>> +    VFIOGroup *group;
> >>>> +    int ret, i;
> >>>> +    bool has_bus_reset = false;
> >>>> +
> >>>> +    ret = vfio_get_hot_reset_info(vdev, &info);
> >>>> +    if (ret < 0) {
> >>> if (ret) {...
> >>>
> >>>> +        goto out;
> >>>> +    }
> >>>> +
> >>>> +    /* List all affected devices by bus reset */
> >>>> +    devices = &info->devices[0];
> >>>> +
> >>>> +    /* Verify that we have all the groups required */
> >>>> +    for (i = 0; i < info->count; i++) {
> >>>> +        PCIHostDeviceAddress host;
> >>>> +        VFIOPCIDevice *tmp;
> >>>> +        VFIODevice *vbasedev_iter;
> >>>> +
> >>>> +        host.domain = devices[i].segment;
> >>>> +        host.bus = devices[i].bus;
> >>>> +        host.slot = PCI_SLOT(devices[i].devfn);
> >>>> +        host.function = PCI_FUNC(devices[i].devfn);
> >>>> +
> >>>> +        /* Skip the current device */
> >>>> +        if (vfio_pci_host_match(&host, &vdev->host)) {
> >>>> +            continue;
> >>>> +        }
> >>>> +
> >>>> +        /* Ensure we own the group of the affected device */
> >>>> +        QLIST_FOREACH(group, &vfio_group_list, next) {
> >>>> +            if (group->groupid == devices[i].group_id) {
> >>>> +                break;
> >>>> +            }
> >>>> +        }
> >>>> +
> >>>> +        if (!group) {
> >>>> +            goto out;
> >>>> +        }
> >>>> +
> >>>> +        /* Ensure affected devices for reset under the same bus */
> >>>> +        QLIST_FOREACH(vbasedev_iter, &group->device_list, next) {
> >>>> +            if (vbasedev_iter->type != VFIO_DEVICE_TYPE_PCI) {
> >>>> +                continue;
> >>>> +            }
> >>>> +            tmp = container_of(vbasedev_iter, VFIOPCIDevice, vbasedev);
> >>>> +            if (vfio_pci_host_match(&host, &tmp->host)) {
> >>>> +                struct VfioDeviceFind find = { .pdev = &tmp->pdev, 
> >>>> .found = false };
> >>>> +
> >>>> +                pci_for_each_bus(bus, find_devices, &find);
> >>>> +                if (!find.found) {
> >>>> +                    goto out;
> >>>> +                }
> >>>> +                /*
> >>>> +                 * When the check device is hotplugged to a higher bus 
> >>>> again,
> >>>> +                 * which would influence the affected device which 
> >>>> enable aer
> >>>> +                 * below the bus.
> >>>> +                 */
> >>>> +                if (tmp->features & VFIO_FEATURE_ENABLE_AER &&
> >>>> +                    find.pbus != bus) {
> >>>> +                    goto out;
> >>>> +                }
> >>> I think what you're trying to do here is assume that if a reset of A
> >>> affects B, then a reset of B affects A, and if B is on a subordinate bus
> >>> from A, then our configuration is broken, right?  Can we really
> >>> guarantee that assumption?  If we had a physical topology that mirrored
> >>> this virtual topology, that wouldn't necessarily be true.  For instance,
> >>> if A was a function of a multi-function device where another function
> >>> was a PCIe upstream switch, B could be subordinate to that switch, so a
> >>> reset of A affects B, but a reset of B doesn't affect A.
> >>>
> >>>> +                break;
> >>>> +            }
> >>>> +        }
> >>>> +    }
> >>>> +
> >>>> +    has_bus_reset = true;
> >>>> +
> >>>> +out:
> >>>> +    vdev->has_bus_reset = has_bus_reset;
> >>> I don't see any value is storing this, it can't be trusted at any point
> >>> in the future.  I think that any time we add a device or think about
> >>> forwarding an AER message to the guest, we need to do a validation pass,
> >>> testing the entire topology.
> >>>
> >>> To do that we'd iterate through every device in every group for PCI
> >>> devices that support AER.  For each one, get the hot reset info for the
> >>> affected device list.  For each affected device, if it's attached to the
> >>> VM, it needs to be on or below the bus of the target device.
> >>> Additionally, we should test all of the non-AER supporting vfio-pci
> >>> devices on or below the target bus to verify they have a reset
> >>> mechanism.  Run that function for each vfio-pci device as it's added,
> >>> regardless of whether it's hotplug.  If the test fails, fail the initfn
> >>> for the current device.  Also run the test prior to AER injection, if it
> >>> fails demote the AER injection to a machine halt as we have now.
> >> I'm worry about is the case that the affected devices belonged to
> >> another groups but when initialize this device the another group
> >> has not been added. it would cause fail the initfn, but the group
> >> maybe added later. so I used the machine done event notifier to
> >> check this case. if we don't do like that. how can we check the
> >> case when all vfio-pci devices initfn ?
> > That's why the initfn test needs to test the entire topology, not just
> > the device being added.  If we have the case you describe where we have
> > two devices in separate groups, we add the first device, test the
> > topology, see that the second device is affected but not yet added and
> > allow AER to be enabled.  When the second device is added, we again test
> > the topology, we see the potential conflict and fail the initfn for the
> > second device if the bus requirements are not met.
> In case that the second device may not be added. in this case the
> first device enable the aer, and pass the validate. how do we know
> the second device if be added ?

Yeah, I see what you're getting at here.  If we have a dual port NIC
with isolation between functions such that each is a separate IOMMU
group, when we add the first device with AER enabled it will fail
because a bus reset affects both groups and we don't yet own the second
group.

My proposal wouldn't provide a way to ever enable AER for these devices.
However, the proposal of using a machine-init-done hook only allows
cold-plug of such devices with AER, hotplug would get the same issue.  I
don't think that sort of asymmetry is supportable either.

We almost need some sort of vfio-group stub driver in qemu that we can
claim ownership of a group without adding any of the devices in the
group to the VM.  Another option might be to make AER a "soft"
requirement, demote it to a VM stop unless the topology allows it.  That
also creates a confusing user scenario that probably looks nearly random
from an outside perspective.  The only other idea I can see would be to
allow some manipulation of iommu groups at the kernel level, perhaps
creating a meta-group composed of one or more iommu groups.  Or maybe a
kernel option that could ignore isolation for specified devices to
broaden the group.  Are we really sure exposing AER to guests is a good
idea?  Requiring guest bus resets to map to host bus rests is really a
mess.  Thanks,

Alex